n, k = map(int, input().split())
a = list(map(int, input().split()))
k -= 1

if a[k] == 0:
    print(0)
    exit(0)

left = 0
right = 0

for i in range(k-1, -1, -1):
    left += a[i]
    if a[i] <= 1:
        break

for i in range(k+1, n):
    right += a[i]
    if a[i] <= 1:
        break

print(max(left, right) + 1 if a[k] == 1 else left + right + a[k])