#include <iostream>
#include <algorithm>
using namespace std;
const int maxn = 100100;
int a[2 * maxn];
int main(){
  int n, m; cin >> n >> m;
  for(int i = 0; i < 2 * n; i++) cin >> a[i];
  sort(a, a + 2 * n);
  int l = 0, r = n, a = 0;
  while(l <= r){
    int mid = (l + r)>>1;
    bool ok = true;
    for(int i = 2 * mid, j = 2 * n - 1; i < j; i++, j--){
      if(::a[i] + ::a[j] < m){
        ok = false;
        break;
      }
    }
    if(ok) r = mid - 1, a = mid;
    else l = mid + 1;
  }
  int ans = 0;
  for(int i = 0, j = 2 * a - 1; i < j; i++, j--) ans = max(ans, ::a[i] + ::a[j]);
  for(int i = 2 * a, j = 2 * n - 1; i < j; i++, j--) ans = max(ans, ::a[i] + ::a[j] - m);
  cout << ans << endl;
  return 0;
}