N, K = map(int, input().split())
A = list(map(int, input().split()))
K -= 1

ans = A[K]
if A[K] != 0:
    l = r = 0
    if K != 0:
        for i in range(K-1, -1, -1):
            l += A[i]
            if A[i] <= 1:
                break
    if K != N-1:
        for i in range(K+1, N):
            r += A[i]
            if A[i] <= 1:
                break
    if A[K] == 1:
        ans += max(l, r)
    else:
        ans += l + r
print(ans)