Don dieu

#include<bits/stdc++.h>
using namespace std;
string s;
long long l,r,n,c,res,a[1000007],b[1000007],val;
int main(){
cin>>n>>c;
cin>>s;
s='1'+s;
for(int i=1;i<=n;i++){
a[i]=a[i-1];
b[i]=b[i-1];
if(s[i]=='a')a[i]++;
if(s[i]=='b')b[i]++;
}
l=1;
for(r=1;r<=n;r++){
if(s[r]=='b')val+=(a[r-1]-a[l-1]);
while(val>c){
if(s[l]=='a')val-=(b[r]-b[l]);
l++;
}
res=max(res,r-l+1);
}
cout<<res;
return 0;
}
//using two pointer.