//https://o...content-available-to-author-only...u.vn/problem/substr1
#include<bits/stdc++.h>
using namespace std;
const long long MaxN = 1e6 + 5;
long long k,prefix[MaxN];
string s;
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cin >> k;
    cin >> s;
    s=" " + s;
    for (long long i=1; i<s.size(); i++)
    {
        prefix[i]=prefix[i-1];
        if(s[i]=='1')
        {
            prefix[i]++;
        }
    }
    long long res=0;
    for (long long i=1; i<s.size(); i++)
    {
        if (prefix[i] - k >= 0)
        {

            long long t1 = lower_bound(prefix, prefix + i, prefix[i] - k) - prefix;
            long long t2 = upper_bound(prefix, prefix + i, prefix[i] - k) - prefix;


            res += (t2 - t1);
        }
    }
    cout << res;
}