#include <bits/stdc++.h>

using namespace std;
#define push_back pb
#ifdef LOCAL
#include "algo/debug.h"
#else
#define debug(...) 42
#endif

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int tt;
  cin >> tt;
  for (int qq = 1; qq <= tt; qq++) {
    cout << "Case #" << qq << ": ";
    int n, k;
    cin >> n >> k;
    if (k % 2 == 1 || k < n - 1) {
      cout << "IMPOSSIBLE" << '\n';
      continue;
    }
    auto GetXY = [&](int id) {
      id -= 1;
      int x = 0, y = 0;
      int sz = n;
      while (sz > 1) {
        if (id < sz - 1) {
          return make_pair(x, y + id);
        }
        id -= sz - 1;
        y += sz - 1;
        if (id < sz - 1) {
          return make_pair(x + id, y);
        }
        id -= sz - 1;
        x += sz - 1;
        if (id < sz - 1) {
          return make_pair(x, y - id);
        }
        id -= sz - 1;
        y -= sz - 1;
        if (id < sz - 1) {
          return make_pair(x - id, y);
        }
        id -= sz - 1;
        x -= sz - 2;
        y += 1;
        sz -= 2;
      }
      return make_pair(x, y);
    };
    auto GetId = [&](pair<int, int> p) {
      int x = p.first;
      int y = p.second;
      int skip = min(min(x, n - 1 - x), min(y, n - 1 - y));
      int res = n * n - (n - 2 * skip) * (n - 2 * skip) + 1;
      if (x == skip) {
        return res + (y - skip);
      }
      res += n - 2 * skip - 1;
      if (n - 1 - y == skip) {
        return res + (x - skip);
      }
      res += n - 2 * skip - 1;
      if (n - 1 - x == skip) {
        return res + (n - 1 - skip) - y;
      }
      res += n - 2 * skip - 1;
      assert(y == skip);
      return res + (n - 1 - skip) - x;
    };
    int low = 1, high = n * n;
    while (low < high) {
      int mid = (low + high + 1) >> 1;
      auto p = GetXY(mid);
      int steps = (mid - 1) + abs(p.first - n / 2) + abs(p.second - n / 2);
      if (steps <= k) {
        low = mid;
      } else {
        high = mid - 1;
      }
    }
    vector<pair<int, int>> shortcuts;
    auto Add = [&](int i, int j) {
      assert(i < j);
      if (i + 1 < j) {
        shortcuts.emplace_back(i, j);
      }
    };
    auto p = GetXY(low);
    while (p.first < n / 2) {
      auto q = p;
      q.first += 1;
      Add(GetId(p), GetId(q));
      p = q;
    }
    while (p.first > n / 2) {
      auto q = p;
      q.first -= 1;
      Add(GetId(p), GetId(q));
      p = q;
    }
    while (p.second < n / 2) {
      auto q = p;
      q.second += 1;
      Add(GetId(p), GetId(q));
      p = q;
    }
    while (p.second > n / 2) {
      auto q = p;
      q.second -= 1;
      Add(GetId(p), GetId(q));
      p = q;
    }
    cout << shortcuts.size() << '\n';
    for (auto& pp : shortcuts) {
      cout << pp.first << " " << pp.second << '\n';
    }
  }
  return 0;
}

C++ Online Compiler

Write, Run & Share C++ code online using OneCompiler's C++ online compiler for free. It's one of the robust, feature-rich online compilers for C++ language, running on the latest version 17. Getting started with the OneCompiler's C++ compiler is simple and pretty fast. The editor shows sample boilerplate code when you choose language as C++ and start coding!

Read inputs from stdin

OneCompiler's C++ online compiler supports stdin and users can give inputs to programs using the STDIN textbox under the I/O tab. Following is a sample program which takes name as input and print your name with hello.

#include <iostream>
#include <string>
using namespace std;

int main() 
{
    string name;
    cout << "Enter name:";
    getline (cin, name);
    cout << "Hello " << name;
    return 0;
}

About C++

C++ is a widely used middle-level programming language.

  • Supports different platforms like Windows, various Linux flavours, MacOS etc
  • C++ supports OOPS concepts like Inheritance, Polymorphism, Encapsulation and Abstraction.
  • Case-sensitive
  • C++ is a compiler based language
  • C++ supports structured programming language
  • C++ provides alot of inbuilt functions and also supports dynamic memory allocation.
  • Like C, C++ also allows you to play with memory using Pointers.

Syntax help

Loops

1. If-Else:

When ever you want to perform a set of operations based on a condition If-Else is used.

if(conditional-expression) {
   //code
}
else {
   //code
}

You can also use if-else for nested Ifs and If-Else-If ladder when multiple conditions are to be performed on a single variable.

2. Switch:

Switch is an alternative to If-Else-If ladder.

switch(conditional-expression){    
case value1:    
 // code    
 break;  // optional  
case value2:    
 // code    
 break;  // optional  
......    
    
default:     
 code to be executed when all the above cases are not matched;    
}

3. For:

For loop is used to iterate a set of statements based on a condition.

for(Initialization; Condition; Increment/decrement){  
  //code  
}

4. While:

While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.

while (condition) {  
// code 
}

5. Do-While:

Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.

do {  
 // code 
} while (condition);

Functions

Function is a sub-routine which contains set of statements. Usually functions are written when multiple calls are required to same set of statements which increases re-usuability and modularity. Function gets run only when it is called.

How to declare a Function:

return_type function_name(parameters);

How to call a Function:

function_name (parameters)

How to define a Function:

return_type function_name(parameters) {  
 // code
}