from operator import itemgetter
from itertools import *
from bisect import *
from collections import *
from heapq import *
import sys

sys.setrecursionlimit(10 ** 6)

def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def SI(): return sys.stdin.readline()[:-1]
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
int1 = lambda x: int(x) - 1
def MI1(): return map(int1, sys.stdin.readline().split())
def LI1(): return list(map(int1, sys.stdin.readline().split()))
p2D = lambda x: print(*x, sep="\n")
dij = [(1, 0), (0, 1), (-1, 0), (0, -1)]

def main():
    n,m=MI()
    a0=II()
    aa=[II() for _ in range(n-1)]
    aa.sort()

    def ok(i):
        s=a0+aa[i]
        cnt=0
        j0=0
        for j1 in range(n-2,-1,-1):
            if j1==i:continue
            while (aa[j0]+aa[j1]<=s or j0==i) and j0<j1:j0+=1
            if j1<=j0:break
            cnt+=1
            j0+=1
        return cnt<m

    l=-1
    r=n-1
    while l+1<r:
        i=(l+r)//2
        if ok(i):r=i
        else:l=i
    if r==n-1:print(-1)
    else:print(aa[r])

main()