#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];
    }
    auto calc = [&] (vector<LL> x, int k) -> LL {
        for (int i = 0; i < n; i++) {
            a[i] += k * i * d;
        }
        sort(a.begin(), a.end());
        return a[n - 1] - a[0];
    };
    LL res = min(calc(a, 1), calc(a, -1));
    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;
}