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

if a[k] == 0:
    print(0)
    exit()
    
right = 0
i = k
for i in range(i+1, n):
    right += a[i]
    if a[i] <= 1:
        break

left = 0
i = k
for i in range(i-1, -1, -1):
    left += a[i]
    if a[i] <= 1:
        break

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