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

ans = 0

l = 0
if k - 2 >= 0:
  for i in range(k - 2, -1, -1):
    l += A[i]
    if A[i] <= 1:
      break

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