from collections import deque
import sys
input = sys.stdin.buffer.readline
sys.setrecursionlimit(10 ** 7)

N, M = map(int, input().split())
A = int(input())
B = [int(input()) for _ in range(N - 1)]
B.sort()


def F(x):
    C = deque()
    for i, b in enumerate(B):
        if i == x:
            continue
        C.append(b)

    score = A + B[x]
    cnt = 0
    while C:
        R = C.pop()
        while C:
            L = C.popleft()
            if R + L > score:
                cnt += 1
                break
    return cnt >= M


l = -1
r = N-1
while (r - l) > 1:
    m = (r + l) // 2
    if F(m):
        l = m
    else:
        r = m


if r == N - 1:
    print(-1)
else:
    print(B[r])