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

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

print(ans)