N, K = map(int, input().split())
X = list(map(int, input().split()))
if X[K-1] == 0:
    print(0)
elif X[K-1] == 1:
    a = 0
    for i in range(K, N):
        a += X[i]
        if X[i] <= 1:
            break
    b = 0
    for i in range(max(0, K-2)):
        b += X[i]
        if X[i] <= 1:
            break
    print(1+max(a, b))
else:
    a = 0
    for i in range(K, N):
        a += X[i]
        if X[i] <= 1:
            break
    b = 0
    for i in range(max(0, K-2), -1, -1):
        b += X[i]
        if X[i] <= 1:
            break
    print(a+b+X[K-1])