from collections import *
from itertools import *
from functools import *
from heapq import *
import sys,math
input = sys.stdin.readline

N,M = map(int,input().split())
A = [int(input()) for _ in range(N)]
a0 = A[0]
A = A[1:]
A.sort()
A = [a0] + A
def is_ok(i):
    
    threshold = A[i]+A[0]
    X = []
    for j in range(1,N):
        if j!=i:
            X.append(A[j])
    X.sort(reverse=True)
    v = deque(X)
    cnt = 0
    while v:
        x = v.popleft()
        while v:
            y = v.pop()
            if x+y>threshold:
                cnt += 1
                break
            
    return cnt<M

if is_ok(1):
    print(A[1])
    exit()
if not is_ok(N-1):
    print(-1)
    exit()
y = N-1
x = 1
while y-x>1:
    mid = (y+x)//2
    if is_ok(mid):
        y = mid
    else:
        x = mid
print(A[y])