N, K = map(int, input().split())
A = list(map(int, input().split()))
left, right = 0, N
K -= 1
for i in range(K - 1, -1, -1):
    if A[i] <= 1:
        left = i
        break
for i in range(K + 1, N):
    if A[i] <= 1:
        right = i + 1
        break
L, R = 0, 0
for i in range(left, K):
    L += A[i]
for i in range(K + 1, right):
    R += A[i]
if A[K] == 0:
    print(0)
elif A[K] == 1:
    print(max(L, R) + 1)
else:
    print(L + R + A[K])