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

left = 0
right = 0

for i in range(K - 1):
    left += A[K - 2 - i]
    if A[K - 2 - i] <= 1:
        break

for i in range(N - K):
    right += A[K + i]
    if A[K + i] <= 1:
        break

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

elif A[K - 1] == 1:
    print(max(left,right) + 1)

else:
    print(left + A[K - 1] + right)