N,K = map(int, input().split())
A = [int(a) for a in input().split()]

if A[K-1] > 0:
    ans = A[K-1]
    c1 = 0
    c2 = 0
    for i in range(K, N):
        c1 += A[i]
        if A[i] < 2:
            break
    for i in range(K-2, -1, -1):
        c2 += A[i]
        if A[i] < 2:
            break
    if A[K-1] == 1:
        ans += max(c1, c2)
    else:
        ans += c1+c2
else:
    ans = 0

print(ans)