#include <bits/stdc++.h>
using namespace std;
const long long LLINF = 1e18 + 7;
const long long INF = 1e9 + 7;
const long long EPS = 1e-9 - 7;
const long long MAX = 1e2 + 7;
const long long MOD = 1e9 + 7;
int n, d, dp[MAX];
string s;
bool can[MAX], flag = false;
int main() {
  memset(dp, MAX, sizeof dp);
  cin >> n >> d >> s;
  for (int i = 0; i < n; i++) {
    if (s[i] == '1') {
      can[i + 1] = true;
    }
  }
  for (int i = n - 1; i >= 1; i--) {
    if (n - i <= d && can[i]) {
      flag = true;
      dp[i] = 1;
    }
  }
  if (!flag) {
    cout << -1 << endl;
    return 0;
  }
  for (int i = n - 1; i >= 1; i--) {
    for (int j = i - 1; j >= 1; j--) {
      if (i - j <= d && can[j] && can[i]) {
        dp[j] = min(dp[j], dp[i] + 1);
      }
    }
  }
  if (dp[1] >= n) {
    cout << -1 << endl;
    return 0;
  }
  cout << dp[1] << endl;
  return 0;
}
