N,K = map(int,input().split())
A = [0]+list(map(int,input().split()))+[0]
l0 = 0
for i in range(K-1,0,-1):
    if A[i]==0:
        l0 = i
        break
l1 = 0
for i in range(K-1,l0,-1):
    if A[i]==1:
        l1 = i
        break
left_sum = sum(A[l1:K])
r0 = N+1
for i in range(K+1,N+1):
    if A[i]==0:
        r0 = i
        break
r1 = N+1
for i in range(K+1,r0):
    if A[i]==1:
        r1 = i
        break
right_sum = sum(A[K+1:r1])
if r1<N+1:
    right_sum += 1
ans = 0
if A[K]==1:
    ans = A[k]+max(left_sum,right_sum)
elif A[K]>1:
    ans = A[K]+left_sum+right_sum
print(ans)