n, m = map(int, input().split())
a = [int(input()) for _ in range(n)]
a0 = a[0]
others = sorted(a[1:])
total_others = len(others)

ans = -1
low = 0
high = total_others - 1

while low <= high:
    mid = (low + high) // 2
    x = others[mid]
    # Create B by removing x at mid position
    B = others[:mid] + others[mid+1:]
    S = a0 + x
    cnt = 0
    l = 0
    r = len(B) - 1
    while l < r:
        if B[l] + B[r] > S:
            cnt += 1
            l += 1
            r -= 1
        else:
            l += 1
    if cnt <= m - 1:
        ans = x
        high = mid - 1
    else:
        low = mid + 1

print(ans if ans != -1 else -1)