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; }