import sys
input = sys.stdin.readline

N,M=map(int,input().split())
A=[int(input()) for i in range(N)]

K=A[0]
B=A[1:]

B.sort()

OK=len(B)
NG=-1

while OK>NG+1:
    mid=(OK+NG)//2

    now=K+B[mid]
    C=B[:mid]+B[mid+1:]

    ind=0
    member=0
    for i in range(len(C)-1,-1,-1):
        while ind<i and C[ind]+C[i]<=now:
            ind+=1

        if ind<i and C[ind]+C[i]>now:
            member+=1
            ind+=1

    #print(mid,now,member,C)

    if member<M:
        OK=mid
    else:
        NG=mid

if OK==len(B):
    print(-1)
else:
    print(B[OK])