import sys
input = sys.stdin.readline

N, K = map(int, input().split())
a = list(map(int, input().split()))
left, right = 0, 0
for i in range(K, N):
    if a[i] < 2:
        right += a[i] == 1
        break
    right += a[i]

for i in range(K-2, -1, -1):
    if a[i] < 2:
        left += a[i] == 1
        break
    left += a[i]

if a[K-1] == 0:
    print(0)
elif a[K-1] == 1:
    print(max(left, right)+1)
else:
    print(left + right + a[K-1])