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

const int N = 2e5 + 5;

int A[N], B[N], n, m;

bool pd(int x) {
	int now = x * 2;
	for(int i = x * 2 + 1; i <= n * 2; i += 2) {
		if(A[i] + A[n * 2 + x * 2 + 1 - i] < m) return 0;
	}
	return 1;
}

int calc(int now) {
	int ans = 0;
	for(int i = now + 1; i <= n * 2; i += 2){
		ans = max(ans, A[i] + A[n * 2 + now + 1 - i] - m);
	}
	for(int i = 1; i <= now; ++ i) {
		ans = max(ans, A[i] + A[now + 1 - i]);
	}
	return ans;
}

int main() {
	cin >> n >> m;
	for(int i = 1; i <= (n << 1); ++ i) scanf("%d", &A[i]);
	sort(A + 1, A + (n * 2) + 1);
	int l = 0, r = n, res = -1;
	while(l <= r) {
		int mid = (l + r) >> 1;
		if(pd(mid)) {
			r = mid - 1;
			res = mid;
		}
		else l = mid + 1;
	}
	cout << calc(res * 2) << endl;
}