from itertools import accumulate
N,K = map(int,input().split())
N += 1
A = [0]+list(map(int,input().split()))
if A[K] == 0:
  print(0)
  exit()
l,r = 1,N
for i in range(K,N):
  if A[i] == 0 or A[i] == 1:
    r = i
    break
for i in range(K,-1,-1):
  if A[i] == 1:
    l = i
    break
AC = list(accumulate(A))
if A[K] == 1:
  ans = max(AC[r]-AC[K-1],AC[K]-AC[l-1])
else:
  ans = AC[r]-AC[l-1]
print(ans)