def main():
    import sys
    input = sys.stdin.readline

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

    K -= 1
    if A[K] == 0:
        print(0)
        exit()

    ans = A[K]
    ans1 = 0
    for k in range(K+1, N):
        ans1 += A[k]
        if A[k] <= 1:
            break
    ans2 = 0
    for k in range(K-1, -1, -1):
        ans2 += A[k]
        if A[k] <= 1:
            break
    if ans > 1:
        print(ans + ans1 + ans2)
    else:
        print(ans + max(ans1, ans2))


if __name__ == '__main__':
    main()