n, k = map(int, input().split())
A = list(map(int, input().split()))
ans = 0
for i in range(k - 1, -1, -1):
  ans += A[i]
  if A[i] <= 1:
    break
  
for i in range(k - 1, n):
  ans += A[i]
  if A[i] <= 1:
    break
    
ans -= A[k - 1]

print(ans)