N, M = list(map(int, input().split(' ')))

A = []
for i in range(N):
    A.append(int(input()))

A = [A[0]] + sorted(A[1:])

left = 0
right = N

while 1 < right - left:
    m = (left + right)//2
    k = 0
    lleft = 1
    rright = N-1
    while lleft < rright:
        if lleft == m:
            lleft = lleft + 1
        elif rright == m:
            rright = rright - 1
        elif A[0] + A[m] < A[lleft] + A[rright]:
            k = k + 1
            lleft = lleft + 1
            rright = rright - 1
        else:
            lleft = lleft + 1
    if k < M:
        right = m
    else:
        left = m

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