def main():
    import sys
    input = sys.stdin.read
    data = input().splitlines()
    
    n, k = map(int, data[0].split())
    light = list(map(int, data[1].split()))
    
    # Convert light array to boolean
    light = [True if x == 1 else False for x in light]
    
    k -= 1  # Adjust k to be zero-indexed
    
    result = 0
    last = -1
    index = k
    
    while index < n:
        while index >= 0 and not light[index]:
            index -= 1
        if index == -1 or index <= last:
            print(-1)
            return
        result += 1
        last = index
        index += k * 2 + 1
    
    if last + k + 1 < n:
        result += 1
        l = False
        for i in range(n - 1, n - k - 2, -1):
            if light[i]:
                l = True
                break
        if not l:
            print(-1)
            return
    
    print(result)

if __name__ == '__main__':
    main()