Count substrings with K 1's (CODEFORCES IMPORTANT)

https://codeforces.com/problemset/problem/165/C

A string is binary, if it consists only of characters "0" and "1".
String v is a substring of string w if it has a non-zero length and can be read starting from some position in string w. For example, string "010" has six substrings: "0", "1", "0", "01", "10", "010". Two substrings are considered different if their positions of occurrence are different. So, if some string occurs multiple times, we should consider it the number of times it occurs.
You are given a binary string s. Your task is to find the number of its substrings, containing exactly k characters "1".

Examples

input
1
1010
output
6

input
2
01010
output
4

input
100
01010
output
0

Note
In the first sample the sought substrings are: "1", "1", "10", "01", "10", "010".
In the second sample the sought substrings are: "101", "0101", "1010", "01010".

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#include<bits/stdc++.h>
using namespace std;

void solve(){
   int k;
   cin>>k;
   string s;
   cin>>s;
   long long int sum=0;
   long long int res=0;
   unordered_map<long long int,long long int> mpp;
   for(auto x: s){
      sum+=(x=='1' ? 1 : 0);
      if(sum==k){
         res++;
      }
      if(mpp.find(sum-k)!=mpp.end()){
         res+=mpp[sum-k];
      }
      mpp[sum]++;
   }
   cout<<res;
}
int main(){
   int TC=1;
   while(TC--){
      solve();
   }
}