include

include

using namespace std;

int pylons(int k, const vector& arr) { int need_power = 0; int count = 0;

while (need_power < arr.size()) {
    bool plant_not_found = true;

    for (int i = min(k - 1, int(arr.size()) - need_power - 1); i >= -min(k - 1, need_power); --i) {
        if (arr[need_power + i] == 1) {
            count++;
            need_power += k + i;
            plant_not_found = false;
            break;
        }
    }

    if (plant_not_found) return -1;
}

return count;

}

int main() { int n, k; cin >> n >> k;

vector<int> arr(n);
for (int i = 0; i < n; ++i) {
    cin >> arr[i];
}

int result = pylons(k, arr);

cout << result << "\n";

return 0;

}