#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, d, steps = 0;
  string s;
  cin >> n >> d >> s;
  for (int x = 0; x < n; steps++) {
    if (x == n - 1) {
      cout << steps;
      break;
    }
    for (int a = d; a + 1; a--) {
      if (!a) {
        cout << -1;
        return 0;
      }
      if (x + a < n && s[x + a] == '1') {
        x += a;
        break;
      }
    }
  }
}
