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

if a[k] == 0:
    print(0)
    exit(0)

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

for i in range(k+1, n):
    ans += a[i]
    if a[i] <= 1:
        break

print(ans)