n, k = map(int, input().split())
k -= 1
a = list(map(int, input().split()))
if a[k] == 0:
    print(0)
    exit()
def findlr(k):
    if k < 0 or k >= n:
        return 0
    if a[k] <= 1:
        return a[k]
    l, r = k, k
    while l - 1 >= 0 and a[l - 1] >= 2:
        l -= 1
    while r + 1 < n and a[r + 1] >= 2:
        r += 1
    if l - 1 >= 0 and a[l - 1] >= 1:
        l -= 1
    if r + 1 < n and a[r + 1] >= 1:
        r += 1
    ans = 0
    for i in range(l, r + 1):
        ans += a[i]
    return ans
if a[k] == 1:
    if findlr(k - 1) > 1 or findlr(k + 1) > 1:
        print(max(findlr(k - 1), findlr(k + 1)))
    else:
        print(max(1, findlr(k - 1) + 1, findlr(k + 1) + 1))
else:
    print(findlr(k))