N, K = map(int, input().split())
A = list(map(int, input().split()))

if A[K - 1] == 0:
    print(0)
    exit()

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

#print(p, q)

if A[K - 1] > 1:
    ans = 0
    for i in range(p, q + 1):
        ans += A[i]
    print(ans)
else:
    ans1 = 0
    for i in range(p, K):
        ans1 += A[i]
    ans2 = 0
    for i in range(K - 1, q + 1):
        ans2 += A[i]
    print(max(ans1, ans2))