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: Accepted)

#include <bits/stdc++.h>
#define rep(i, a, b) for(int i = (a); i <= (b); i++)
#define per(i, a, b) for(int i = (a); i >= (b); i--)
#define pb push_back
using namespace std;
const int N = 300010;
int n, m, l, r, mid, mx;
int a[N + N];

template<typename T> void gi(T &x) {
	x = 0; register char c = getchar();
	for(; c < '0' || c > '9'; c = getchar());
	for(; c >= '0' && c <= '9'; c = getchar()) x = x * 10 + (c ^ 48);
}
bool check(int x) {
	rep(i, 1, n - x) if(a[x + x + i] + a[n + n - i + 1] < m) return 0; return 1;
}
int main() {
	gi(n), gi(m); rep(i, 1, n + n) gi(a[i]);
	sort(a + 1, a + n + n + 1);
	l = 0, r = n, mid = 0;
	for(; l < r;) {
		mid = (l + r) >> 1;
		if(check(mid)) r = mid;
		else l = mid + 1;
	}
	rep(i, 1, r) mx = max(mx, (a[i] + a[r + r - i + 1]) % m);
	rep(i, 1, n - r) mx = max(mx, (a[r + r + i] + a[n + n - i + 1]) % m);
	printf("%d\n", mx);
	return 0;
}