import sys
input = sys.stdin.readline

N, K = map(int, input().split())
Alist = list(map(int, input().split()))
left, right = 0, 0
for i in reversed(range(K-1)):
    A = Alist[i]
    if A == 0:
        break
    if A == 1:
        left += 1
        break
    else:
        left += A

for i in range(K, N):
    A = Alist[i]
    if A == 0:
        break
    if A == 1:
        right += 1
        break
    else:
        right += A
now = Alist[K-1]
if now == 0:
    print(0)
if now == 1:
    print(max(left, right)+1)
if now >= 2:
    print(left+right+now)