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

ll n, k;
ll pre[100005];
string s;

ll bs1(ll lo){
    ll l = lo, r = n, mid;
    while (l <= r){
        mid = (l + r) / 2;
        if (pre[mid] - pre[lo-1] <= k){
            l = mid + 1;
        } else {
            r = mid - 1;
        }
    }
    return r;
}

ll bs2(ll lo){
    ll l = lo, r = n, mid;
    while (l <= r){
        mid = (l + r) / 2;
        if (pre[mid] - pre[lo-1] >= k){
            r = mid - 1;
        } else {
            l = mid + 1;
        }
    }
    return l;
}

signed main(){
    ll i, res;
    cin >> k >> s;
    n = s.size();
    s = " " + s;
    pre[0] = 0;
    for (i = 1; i <= n; i++){
        pre[i] = pre[i-1] + s[i] - '0';
    }
    res = 0;
    for (i = 1; i <= n; i++){
        res += bs1(i) - bs2(i) + 1;
    }
    cout << res;
}