n,k = map(int,input().split())
a = [int(i) for i in input().split()]

if a[k-1] == 0:
    print(0)
    exit()

l = 0
now = k-2

for i in range(n):
    if now != -1:
        if a[now] > 1:
            l += a[now]
        else:
            l += a[now]
            break
        now -= 1

r = 0
now = k

for i in range(n):
    if now != n:
        if a[now] > 1:
            r += a[now]
        else:
            r += a[now]
            break
        now += 1
    #print(r)

if a[k-1] == 1:
    print(max(r,l))
else:
    print(r+l+a[k-1])