#include <bits/stdc++.h>
using namespace std;
using LL = long long;
inline void solve() {
    int n;
    LL d;
    cin >> n >> d;
    
    vector<LL> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
        a[i] -= i * d;
    }
    sort(a.begin(), a.end());
    
    LL res = a[n - 1] - a[0];
    cout << res / 2;
    if (res % 2 == 1) {
        cout << ".5";
    } else {
        cout << ".0";
    }
    cout << '\n';

    
}

signed main() {
    cin.tie(0)->sync_with_stdio(0);
    int t = 1;

    while (t--)
        solve();
    return 0;
}