import sys

sys.setrecursionlimit(10 ** 8)

input = sys.stdin.readline


def main():
    N, K = [int(x) for x in input().split()]
    A = [int(x) for x in input().split()]

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

    ans1 = 0
    for a in A[K:]:
        if a < 2:
            ans1 += a
            break
        ans1 += a

    ans2 = 0
    for i in range(K - 2, -1, -1):
        a = A[i]
        if a < 2:
            ans2 += a
            break
        ans2 += a

    if A[K - 1] == 1:
        print(max(ans1, ans2) + A[K - 1])
    else:
        print(ans1 + ans2 + A[K - 1])



    
    
    

if __name__ == '__main__':
    main()