#include <bits/stdc++.h>
using namespace std;
template <class c>
struct rge {
  c b, e;
};
template <class c>
rge<c> range(c i, c j) {
  return rge<c>{i, j};
}
template <class c>
auto dud(c* x) -> decltype(cerr << *x, 0);
template <class c>
char dud(...);
struct debug {
  template <class c>
  debug& operator<<(const c&) {
    return *this;
  }
};
const long long int N = 5e5 + 5;
const long long int INF = 1e18;
long long int mod = 1e9 + 7;
long long int dx[4] = {0, 0, +1, -1};
long long int dy[4] = {+1, -1, 0, 0};
long long int po(long long int a, long long int b) {
  a %= mod;
  long long int res = 1;
  while (b > 0) {
    if (b & 1) res = res * a % mod;
    a = a * a % mod;
    b >>= 1;
  }
  return res;
}
long long int vis[N];
void solve() {
  long long int n, r;
  cin >> n >> r;
  vector<long long int> v(n, 0);
  for (long long int i = 0; i < n; i++) {
    cin >> v[i];
  }
  long long int ct = 0;
  for (long long int i = 0; i < n; i++) {
    if (vis[i]) continue;
    ct++;
    long long int cur = -1;
    for (long long int j = min(n - 1, i + r - 1); j >= max(0ll, i - r + 1);
         j--) {
      if (v[j]) {
        cur = j;
        break;
      }
    }
    if (cur == -1) {
      cout << -1 << "\n";
      return;
    }
    for (long long int j = min(n - 1, cur + r - 1); j >= max(0ll, cur - r + 1);
         j--)
      vis[j] = 1;
  }
  cout << ct << "\n";
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long long int t = 1;
  while (t--) solve();
  return 0;
}
