def pylons(k, arr):
    n = len(arr)
    plants = 0
    i = 0

    while i < n:
        found = False
        for j in range(min(i + k - 1, n - 1), max(-1, i - k), -1):
            if arr[j] == 1:
                plants += 1
                i = j + k
                found = True
                break

        if not found:
            return -1

    return plants

def main():
    n, k = map(int, input().split())
    arr = list(map(int, input().split()))

    result = pylons(k, arr)
    print(result)

if __name__ == "__main__":
    main()