#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pii pair<ll, ll>
#define fi first
#define sec second
#define ld long double
#pragma GCC optimize ("Ofast")
#pragma GCC optimize ("unroll-loops")
const int MAXN = 5e5;
const ll INF = 1e18;
const int MOD = 1e9 + 7;
ll A[MAXN + 5];
int main(){
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int tc = 1;
// cin >> tc;
for(;tc--;){
ll N, T; cin >> N >> T;
for(int i = 1; i <= N; i++){
cin >> A[i];
A[i] %= T;
}
sort(A + 1, A + 1 + N);
ll MX = 0;
for(int i = 2; i <= N; i++){
MX = max(MX, A[i] - A[i - 1] - 1);
}
MX = max(MX, T + A[1] - A[N] - 1);
// cout << MX << "\n";
cout << (T - MX) / 2 << "\n";
}
}
/*
*/
