#include <bits/stdc++.h>
using namespace std;
#define F0R(i, n) for (int i = 0; i < n;i++)
#define FOR(i,j, n) for (int i = j; i < n;i++)
template<typename T>
using V = vector<T>;
using vi = V<int>;
int main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr);
int n, t; cin >> n >> t;
vi arr(n);
F0R(i, n) {
cin >> arr[i];
arr[i] %= t;
}
std::sort(arr.begin(), arr.end());
int MAX_dIST = 0;
FOR(i, 1, n) {
MAX_dIST = max(MAX_dIST, abs(arr[i] - arr[i - 1]));
}
MAX_dIST = max(MAX_dIST, t - (arr[n - 1] - arr[0]));
if (t == 1) t--;
cout << (t - MAX_dIST + 1) / 2;
return 0;
}
