N,K = map(int, input().split())
A = list(map(int, input().split())) 
r=K-1
l=K-1
if A[K-1]>=2:
    while l>=1 and A[l]>=2 and A[l-1]>=1:
        l-=1
    while r<N-1 and A[r]>=2 and A[r+1]>=1:
        r+=1
    print(sum(A[l:r+1]))
elif A[K-1]==0:
    print(0)
else:
    A[K-1]+=1
    while l>=1 and A[l]>=2 and A[l-1]>=1:
        l-=1
    while r<N-1 and A[r]>=2 and A[r+1]>=1:
        r+=1
    print(max(sum(A[l:K]),sum(A[K-1:r+1]))-1)