N, K = map(int, input().split())
K -= 1
A = list(map(int, input().split()))
j = K
cl = 0
while 0 < j:
    j -= 1
    cl += A[j]
    if A[j] <= 1:
        break
j = K
cr = 0
while j < N - 1:
    j += 1
    cr += A[j]
    if A[j] <= 1:
        break

if A[K] == 0:
    print(0)
elif A[K] == 1:
    print(A[K] + max(cl, cr))
else:
    print(A[K] + cl + cr)