#include<bits/stdc++.h>
//#include<bits/extc++.h>
using namespace std;
//using namespace __gnu_pbds;
#define S second
#define F first
#define ll long long
//#pragma GCC optimize("Ofast, unroll-loop")
//#pragma GCC target("avx,avx2")
#pragma GCC optimize("O3")
const int inf=0x3f3f3f3f;
const ll inff=0x3f3f3f3f3f3f3f3f;
const int X=1000000007;
int a[500005];
signed main(){
ios::sync_with_stdio(false), cin.tie(nullptr);
int n, t; cin >> n >> t;
for(int i=1 ; i<=n ; i++) cin >> a[i];
for(int i=1 ; i<=n ; i++) a[i]%=t;
sort(a+1,a+n+1);
int mx=t-a[n]+a[1];
for(int i=1 ; i<n ; i++) mx=max(mx,a[i+1]-a[i]);
cout << (t-mx+1)/2 << '\n';
return 0;
}
