n, k = map(int, input().split())
A = list(map(int, input().split()))

k -= 1
if A[k] == 0:
    print(0)
    exit()
l, r = k - 1, k + 1
while l >= 0:
    if A[l] <= 1:
        break
    l -= 1
if l == -1:
    l += 1
while r < n:
    if A[r] <= 1:
        break
    r += 1
if r == n:
    r -= 1

print(sum(A[l:r + 1]))