#include <bits/stdc++.h>
#define int long long
using namespace std;
int INF = 1LL<<30;
signed main() {
#ifndef ONLINE_JUDGE
// for getting input from input.txt
//freopen("input.txt", "r", stdin);
// for writing output to output.txt
//freopen("output.txt", "w", stdout);
#endif
/*#ifdef ONLINE_JUDGE
ios_base::sync_with_stdio(false);
cin.tie(NULL);
#endif*/ //fast IO
int n,t; cin >> n >> t;
vector<int> r(n);
for (int i=0; i<n; i++) {
int x; cin >> x; r[i] = x%t;
}
sort(r.begin(), r.end()); r.push_back(r.front()+t);
int ma = -1;
for (int i=0; i<n; i++) {
ma=max(ma,r[i+1]-r[i]);
}
int sub = ((ma)-(t%2))/2;
int start = t/2;
int ans = start-sub;
cout << ans << endl;
}
