#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define MOD 998244353
void solve()
{
	ll n, t;
	cin >> n >> t;
	vector<ll> a(n);
	for (int i = 0; i < n; i++)
	{
		cin >> a[i];
		a[i] %= t;
	}
	sort(a.begin(), a.end());
	ll mx = (t - a[n - 1] + a[0]) / 2;
	for (int i = 1; i < n; i++)
	{
		mx = max(mx, (a[i] - a[i - 1]) / 2);
	}
	cout << mx << '\n';
}
int main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	int tests = 1;
	// cin >> tests;
	for (int i = 1; i <= tests; i++)
		solve();
}
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