#include<bits/stdc++.h>

using namespace std;

typedef long long ll;

const int maxn=1000100;

const int INF=(1<<29);

int k;

ll sum[maxn];

char s[maxn];

void solve(){

    scanf("%s",s+1);

    int n=strlen(s+1);

    sum[0]=0;

    for(int i=1;i<=n;i++) sum[i]=sum[i-1]+s[i]-'0';

    ll res=0;

    for(int i=1;i<=n;i++){

        int l=lower_bound(sum+i,sum+n+1,k+sum[i-1])-sum;

        int r=upper_bound(sum+i,sum+n+1,k+sum[i-1])-sum;

        if(l==n+1) break;

        res+=r-l;

    }

    cout<<res<<endl;

}

int main(){

    // freopen("in.txt","r",stdin);

    while(cin>>k) solve();

    return 0;

}