n, k = map(int, input().split())
A = list(map(int, input().split()))

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

A = [0] + A + [0]
#print(A)

for i in range(k):
    if A[i] == 0:
        p = i
        q = 0
    elif A[i] == 1:
        p = i
        q = 1

for i in reversed(range(k+1, n+2)):
    if A[i] == 0:
        r = i
        s = 0
    elif A[i] == 1:
        r = i
        s = 1

if A[k] >= 2:
    #print(p, q, r, s)
    ans = sum(A[p:k])+A[k]+sum(A[k+1:r+1])
    print(ans)
else:
    ans += max(sum(A[p:k]), sum(A[k+1:r+1])) + A[k]
    print(ans)