n,k = map(int,input().split())
A = list(map(int,input().split()))
k -= 1
if A[k] == 0:
    print(0)
else:
    l,r = 0,0
    for i in range(k-1, -1, -1):
        l += A[i]
        if A[i] <= 1:
            break
    
    for i in range(k+1, n):
        r += A[i]
        if A[i] <= 1:
            break
    
    if A[k]==1:
        print(max(l, r) + 1)
    else:
        print(l + r + A[k])