#include<iostream>
#include<vector>
using namespace std;
int main() {
int n, t;
cin >> n >> t;
vector<int> pref(n);
int cold = 0;
for (int i = 0; i < n; i++) {
cin >> pref[i];
if (pref[i] < pref[cold]) cold = i;
}
for (int i = 0; i < n; i++) {
pref[i] = pref[i] - (t * ((pref[i] - cold) / t));
}
sort(pref.begin(), pref.end());
cold = 0;
int hot = n - 1;
int best = pref[hot] - pref[cold];
while (hot > 0) {
pref[hot] -= t;
if (pref[hot] < pref[cold]) cold = hot;
hot--;
best = max(best, pref[hot] - pref[cold]);
}
pref[hot] -= t;
hot = pref[0];
cold = pref[0];
for (int i = 0; i < n; i++) {
hot = max(hot, pref[i]);
cold = min(cold, pref[i]);
}
best = min(best, hot - cold);
cout << (best + 1) / 2 << endl;
return 0;
}
