N,K=map(int,input().split)
A=list(map(int,input().split))
l,r=0
if K==0:
    ans=0
elif K>2:
    for i in range(K-1,-1):
        l+=A[i]
        if A[i]<=1:
            break
    for i in range(K+1):
        r+=A[i]
        if A[i]<=1:
            break
elif K==1:
    ans=max(l.r)
else:
    ans=l+r
print(ans)