#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
/**
* JOI 2024 Final Round - Problem A: Room Temperature
* The problem asks to minimize the maximum discomfort index.
* Discomfort is distance to the set {Ai - k*T | k >= 0}.
* This is minimized by treating Ai modulo T as points on a circle.
*/
int main() {
// Optimize standard I/O
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int n;
long long t_val;
if (!(cin >> n >> t_val)) return 0;
// We only care about the remainder of appropriate temperatures modulo T
vector<long long> b(n);
for (int i = 0; i < n; ++i) {
long long a;
cin >> a;
b[i] = a % t_val;
}
// Sort the points to easily find gaps between them
sort(b.begin(), b.end());
long long max_gap = 0;
// Calculate gaps between adjacent points
for (int i = 0; i < n - 1; ++i) {
max_gap = max(max_gap, b[i + 1] - b[i]);
}
// Calculate the "wrap-around" gap between the last and first points
// This is the distance from the largest value back to the smallest through the boundary T
max_gap = max(max_gap, t_val - (b.back() - b.front()));
// The minimum window covering all points on the circle is (T - largest gap).
long long d = t_val - max_gap;
// The minimum unpleasantness U is the radius needed to cover that window.
// U = ceil(d / 2). Since we are using integer division:
long long ans = (d + 1) / 2;
cout << ans << endl;
return 0;
}
