#!/usr/bin/env python3
# %%
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
from collections import deque

# %%
N, M, *A = map(int, read().split())

# %%
me = A[0]
A = sorted(A[1:])


# %%
def can_win(n):
    # n番目の人と組んだ場合の結果
    B = A[:]
    team_pt = B[n] + me
    del B[n]
    B = deque(B)
    pair_cnt = 0
    while len(B) >= 2:
        if B[0] + B[-1] > team_pt:
            pair_cnt += 1
            B.popleft()
            B.pop()
        else:
            B.popleft()
    return pair_cnt <= M - 1


# %%
left = -1
right = N - 1
while left + 1 < right:
    x = (left + right) // 2
    if can_win(x):
        right = x
    else:
        left = x


# %%
print(-1 if right == N - 1 else A[right])