Count of substrings of a binary string containing K ones

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

int main()
{
    string s; cin>>s; int k; cin>>k;
    int n=s.length();
    
    int freq[n+1];
    int ans=0,count1=0;
    
    freq[0]=1;
    
    for(int i=0;i<n;i++)
    {
        count1+=(s[i]-'0');
        
        if(count1>=k)
        {
            ans+=freq[count1-k];
        }
        
        freq[count1]++;
    }
    
    cout<<ans<<endl;
    
    
    return 0;
    
    
}