N, K = map(int,input().split())
A = list(map(int,input().split()))
if A[K-1] == 0:
    print(0)
elif A[K-1] == 1:
    now = K-2
    l = 0
    while now >= 0:
        if A[now] >= 2:
            l += A[now]
        elif A[now] == 1:
            l += 1
            break
        else:
            break
        now -= 1
    r = 0
    now = K
    while now < N:
        if A[now] >= 2:
            r += A[now]
        elif A[now] == 1:
            r += 1
            break
        else:
            break
        now += 1
    print(max(l,r)+1)
else:
    now = K-2
    l = 0
    while now >= 0:
        if A[now] >= 2:
            l += A[now]
        elif A[now] == 1:
            l += 1
            break
        else:
            break
        now -= 1
    r = 0
    now = K
    while now < N:
        if A[now] >= 2:
            r += A[now]
        elif A[now] == 1:
            r += 1
            break
        else:
            break
        now += 1
    print(l+r+A[K-1])