N, M = map(int, input().split())
A0 = int(input())
A = sorted(int(input()) for _ in range(N - 1))
l, r = -1, N - 1
while r - l > 1:
    m = (r + l) // 2
    am = A[m] + A0
    pair_l = 0
    ct = 0
    for pair_r in range(N - 2, -1, -1):
        if pair_r == m:
            continue
        while pair_l < pair_r and (A[pair_l] + A[pair_r] <= am or pair_l == m):
            pair_l += 1
        if pair_l >= pair_r:
            break
        pair_l += 1
        ct += 1
    if ct >= M:
        l = m
    else:
        r = m
ans = A[r] if r < N - 1 else -1
print(ans)