n, k = map(int, input().split())
alst = list(map(int, input().split()))
k -= 1
if alst[k] == 0:
    print(0)
    exit()

pos_l = k - 1
while 1:
    if alst[pos_l] < 2 or pos_l == 0:
        break
    pos_l -= 1

pos_r = k + 1
while 1:
    if alst[pos_r] < 2 or pos_r == n - 1:
        break
    pos_r += 1
pos_r += 1

l_sum = sum(alst[pos_l:k])
r_sum = sum(alst[k + 1:pos_r])

if alst[k] == 1:
    print(max(l_sum, r_sum) + 1)
else:
    print(l_sum + r_sum + alst[k])