n,m=map(int,input().split())
A=[int(input()) for _ in range(n)]
A=[A[0]]+sorted(A[1:])

def check(x):
    k_score=A[0]+A[x]
    l,r=1,n-1
    cnt=0
    while l<r:
        if l==x:
            l+=1
        elif r==x:
            r-=1
            
        elif A[l]+A[r]>k_score:
            l+=1
            r-=1
            cnt+=1
        elif A[l]+A[r]<=k_score:
            l+=1
        
    return cnt<m
            
ng,ok=0,n
while abs(ok-ng)>1:
    mid=(ok+ng)//2
    if check(mid):ok=mid
    else:ng=mid

print(A[ok] if ok!=n else -1)