#include <bits/stdc++.h>
using namespace std;
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T>
void __print(const T &x) {
  long long f = 0;
  cerr << '{';
  for (auto &i : x) cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cerr << ", ";
  _print(v...);
}
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long long n, r;
  cin >> n >> r;
  vector<long long> a(n);
  long long x = -1;
  for (long long i = 0; i < n; i++) {
    cin >> a[i];
  }
  long long ans = 0;
  while (x < n - 1) {
    long long temp = -1;
    for (long long i = n - 1; i > max(-1LL, x - r + 1); i--) {
      if (a[i] == 1 && i - x <= r) {
        temp = i;
        break;
      }
    }
    if (temp == -1) {
      cout << "-1\n";
      return 0;
      ;
    }
    ans++;
    x = temp + r - 1;
  }
  cout << ans;
}
