#include <bits/stdc++.h>
#include <experimental/random>
#include <random>
//#include <ext/pb_ds/assoc_container.hpp>
//using namespace __gnu_pbds;
using namespace std;
using ld = long double;
using ll = long long;
const ll INF = 1e18, MOD = 1e9 + 7;
void solve();
signed main() {
#ifdef LOCAL
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
ios::sync_with_stdio(false);
cin.tie(0);
int q = 1;
//cin >> q;
while (q--) {
solve();
}
}
void solve() {
int n, t; cin >> n >> t;
vector<int> a(n);
for (int i = 0; i < n; i++) {
cin >> a[i];
a[i] %= t;
}
int l = -1, r = t / 2;
while (r - l > 1) {
int mid = (l + r) / 2;
vector<pair<int, int>> seg;
for (int i = 0; i < n; i++) {
if (a[i] - mid < 0) {
seg.emplace_back(0, -1);
seg.emplace_back(a[i] + mid, 1);
seg.emplace_back(a[i] - mid + t, -1);
seg.emplace_back(t - 1, 1);
} else if (a[i] + mid >= t) {
seg.emplace_back(a[i] - mid, -1);
seg.emplace_back(t - 1, 1);
seg.emplace_back(0, -1);
seg.emplace_back(a[i] + mid - t, 1);
} else {
seg.emplace_back(a[i] - mid, -1);
seg.emplace_back(a[i] + mid, 1);
}
}
sort(seg.begin(), seg.end());
int cnt = 0; bool flag = false;
for (auto [idx, ev] : seg) {
cnt += ev;
if (cnt == -n) flag = true;
}
if (flag) {
r = mid;
} else {
l = mid;
}
}
cout << r;
}
