N, K = list(map(int, input().split()))
A_list = list(map(int, input().split()))

if A_list[K-1] == 0:
    print(0)
else:
    left = 0
    right = 0
    position = K-1
    while True:
        # 左
        position -= 1
        if position >= 0:
            if A_list[position] <= 1:
                left += A_list[position]
                break
            else:
                left += A_list[position]
        else:
            break
    position = K-1
    while True:
        # 右
        position += 1
        if position <= N-1:
            if A_list[position] <= 1:
                right += A_list[position]
                break
            else:
                right += A_list[position]
        else:
            break
    if A_list[K-1] == 1:
        print(max([left, right])+1)
    else:
        print(left+right+A_list[K-1])