import sys
input = sys.stdin.readline

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

if A[K-1] == 0:
    ans = 0
else:
    ans = A[K-1]
    l = K-2
    while l >= 0:
        ans += A[l]
        if A[l] < 2:
            break
        l -= 1
    
    r = K
    while r < N:
        ans += A[r]
        if A[r] < 2:
            break
        r += 1

print(ans)