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

using ll = long long;
using pii = pair<int, int>;
using vpi = vector<pii>;
using vi = vector<int>;
using vll = vector<long long>;
using mpii = map<int, int>;
using mpll = map<ll, ll>;
using db = long double;

#define pb push_back
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define lb lower_bound
#define ub upper_bound
#define make_unique(x)                                                         \
  sort(all((x)));                                                              \
  (x).resize(unique(all((x))) - (x).begin())
#define ceil(a, b) ((a) + (b)-1) / (b)

const int MOD = (int)1e9 + 7;
const db PI = acos((db)-1);
const int dx[4]{1, 0, -1, 0};
const int dy[4]{0, 1, 0, -1};

template <typename K, typename V>
ostream &operator<<(ostream &os, const pair<K, V> &p);
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec);
template <typename K, typename V>
ostream &operator<<(ostream &os, const map<K, V> &m);
template <typename K, typename V>
ostream &operator<<(ostream &os, const unordered_map<K, V> &m);
template <typename T> ostream &operator<<(ostream &os, const set<T> &s);
template <typename T>
ostream &operator<<(ostream &os, const unordered_set<T> &s);

template <typename K, typename V>
ostream &operator<<(ostream &os, const pair<K, V> &p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}

template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "{";
  for (size_t i = 0; i < vec.size(); ++i) {
    if (i > 0)
      os << ", ";
    os << vec[i];
  }
  os << "}";
  return os;
}

template <typename K, typename V>
ostream &operator<<(ostream &os, const map<K, V> &m) {
  os << "{";
  for (const auto &p : m) {
    os << p.first << ": " << p.second << ", ";
  }
  os << "}";
  return os;
}

template <typename K, typename V>
ostream &operator<<(ostream &os, const unordered_map<K, V> &m) {
  os << "{";
  for (const auto &p : m) {
    os << p.first << ": " << p.second << ", ";
  }
  os << "}";
  return os;
}

template <typename T> ostream &operator<<(ostream &os, const set<T> &s) {
  int i = 0;
  os << "{";
  for (const auto &e : s) {
    if (i > 0)
      os << ", ";
    os << e;
    i++;
  }
  os << "}";
  return os;
}

template <typename T>
ostream &operator<<(ostream &os, const unordered_set<T> &s) {
  int i = 0;
  os << "{";
  for (const auto &e : s) {
    if (i > 0)
      os << ", ";
    os << e;
    i++;
  }
  os << "}";
  return os;
}

void dbg() { cerr << "\n"; }

template <typename T, typename... TS> void dbg(T val, TS... vals) {
  cerr << val << " ";
  dbg(vals...);
}

/* stuff you should look for:
  ---------------------------
   * special cases (n=1?)
   * int overflow, array bounds
   * do smth instead of nothing and stay organized
   * WRITE STUFF DOWN
   * DON'T GET STUCK ON ONE APPROACH
 */

void solve() {
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  int cur = 0;
  int ans = 0;
  if (k == 1) {
    for (int i = 0; i < n; i++) {
      while (s[i] == '1') {
        ans++;
        i++;
      }
      dbg(ans);
      if (ans == n) {
        cout << ans - 1 << '\n';
      } else {
        cout << -1 << '\n';
      }
      return;
    }
  }
  while (true) {
    int tmp = cur + 1;
    bool ok = false;
    for (int j = tmp + 1; j < min(n, tmp + k); j++) {
      dbg(j);
      if (s[j] == '1') {
        ok = true;
        cur = j;
      }
    }
    if (!ok) {
      cout << -1;
      return;
    }
    ans++;
    if (cur + 1 == n) {
      break;
    }
  }
  cout << ans << '\n';
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  solve();
}