Fibonacci code in c++

#include <iostream>
using namespace std;

int main() {
int n, a = 1, b = 2, c = 0;
cin >> n;
if (n == 1) {
cout << a;
}
else if (n == 2) {
cout << a + b;
}
else {
while (c < n) {
c = a + b;
a = b;
b = c;
}
cout << c + a;
}
return 0;
}