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

pos_l = k
while alst[pos_l] > 1 and pos_l != -1:
    pos_l -= 1
    if pos_l == -1:
        break

pos_r = k
while alst[pos_r] > 1 and pos_r != n:
    pos_r += 1
    if pos_r == n:
        break
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])