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

if A[k] == 0:
    print(0)
elif A[k] == 1:
    print(max(l,r)+1)
else:
    print(l+r+A[k])