#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pb push_back
#define mp make_pair
#define pii pair<int, int>
#define fi first
#define se second
void solve() {
int n, t;
cin >> n >> t;
vector<int> v(1 + n, 0);
set<int> s;
for(int i = 1; i <= n; i++) {
cin >> v[i];
s.insert(v[i] % t);
}
vector<int> w;
for(int x : s) {
w.pb(x);
}
w.pb(w[0] + t);
int maxi = 0;
for(int i = 0; i < w.size() - 1; i++) {
maxi = max(maxi, w[i + 1] - w[i]);
}
int rem = t - maxi;
int ans = (rem + 1) / 2;
cout << ans << "\n";
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
// int T = 1; cin >> T; while(T--)
solve();
}
