n, k = map(int, input().split())
a = [int(x) for x in input().split()]

if a[k-1] == 0:
    ans = 0
else:
    for i in range(min([k,n-1]), n):
        if a[i] == 1:
            k1 = i
            break
        elif a[i] == 0:
            k1 = i-1
            break
        if i == n-1:
            k1 = n-1
    for i in range(max([k-2, 0]), -1, -1):
        if a[i] == 1:
            k2 = i
            break
        elif a[i] == 0:
            k2 = i+1
            break
        if i == 0:
            k2 = 0
    if a[k-1] != 1:
        ans = sum(a[k2:k1+1])
    else:
        ans = max([sum(a[k2:k]), sum(a[k-1:k1+1])])
print(ans)