Problem:
Score : 1200 points

Problem Statement

Let M be a positive integer.

You are given 2 N integers a_1, a_2, \ldots, a_{2 N}, where 0 \leq a_i < M for each i.

Consider dividing the 2 N integers into N pairs.
Here, each integer must belong to exactly one pair.

We define the ugliness of a pair (x, y) as (x + y) \mod M.
Let Z be the largest ugliness of the N pairs. Find the minimum possible value of Z.

Constraints

All values in input are integers.

1 \leq N \leq 10^5

1 \leq M \leq 10^9

0 \leq a_i < M

Input

Input is given from Standard Input in the following format:

N M
a_1 a_2 \cdots a_{2N}

Output

Print the minimum possible value of Z, where Z is the largest ugliness of the N pairs.

Sample Input 1

3 10
0 2 3 4 5 9

Sample Output 1

5

One solution is to form pairs (0, 5), (2, 3) and (4, 9), with ugliness 5, 5 and 3, respectively.

Sample Input 2

2 10
1 9 1 9

Sample Output 2

0

Pairs (1, 9) and (1, 9) should be formed, with ugliness both 0.

Solution in C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <bits/stdc++.h>
using namespace std;

#define f(i,a,b) for (int i = a; i < b; i++)

int main() {
  	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
  
  int n,m; cin>>n>>m;
  int a[2*n];
  f(i,0,2*n) cin>>a[i];
  sort(a,a+2*n);
  int lo = 0, hi = n-1;
  auto check = [&](int x) {
  	f(i,0,2*x) if (a[i]+a[2*x-1-i] >= m) 
     	return 0;
    return 1;
  };
  while (lo<=hi) {
    int mid = (lo+hi)/2;
  	if (check(mid)) hi = mid-1;
    else lo = mid+1;
  }
  // lo is now the breakpoint
  int ans = 0;
  f(i,0,2*lo) ans = max(ans,(a[i]+a[2*lo-1-i])%m);
  f(i,2*lo,2*n) ans = max(ans,(a[i]+a[2*n-1-(i-2*lo)])%m);
  cout << ans << endl;
  	return 0;
}