#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1e6;
int pref[MAXN + 1];
string S;
int n, k;
void fast_io() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
}
pair<int, int> query(int s) {
  int lo = s, hi = n;
  int res = -1;
  while (lo <= hi) {
    int mid = (lo + hi) >> 1;
    if (pref[mid] - pref[s - 1] >= k)
      res = mid, hi = mid - 1;
    else
      lo = mid + 1;
  }
  if (res == -1 or (pref[res] - pref[s - 1]) != k) return make_pair(-1, -1);
  pair<int, int> p = make_pair(res, -1);
  lo = res, hi = n;
  res = -1;
  while (lo <= hi) {
    int mid = (lo + hi) >> 1;
    if (pref[mid] - pref[s - 1] <= k)
      res = mid, lo = mid + 1;
    else
      hi = mid - 1;
  }
  p.second = res;
  return p;
}
int main() {
  fast_io();
  cin >> k >> S;
  n = S.length();
  S = '$' + S;
  for (int i = 1; i <= n; i++) {
    pref[i] = (S[i] - '0') + pref[i - 1];
  }
  long long ans = 0;
  for (int i = 1; i <= n; i++) {
    pair<int, int> p = query(i);
    if (p.first == -1) continue;
    ans += (p.second - p.first + 1);
  }
  cout << ans << '\n';
  return 0;
}
