#include <bits/stdc++.h>
using namespace std;
using ll = long long;
//#define int ll
using P = pair<int, int>;
#define all(x) x.begin(), x.end()
#define rep(i, l, n) for (int i = l; i < (n); ++i)
#define sz(x) (int)x.size()
const char nl = '\n';
const int mod = 998244353;
const int inf = 1e9;
void solve() {
int n, t; cin >> n >> t;
int mn = inf, mx = 0;
rep(i, 0, n) {
int x; cin >> x;
mn = min(mn, x);
mx = max(mx, x);
}
int d = (mx-mn)/t;
mx -= d*t;
cout << (mx-mn+1)/2;
}
int32_t main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
solve();
return 0;
}
