#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
//typedef long double ld;
int n;
ll d,a[1000005];
long long llINF = 1e18;

int main() {
    ios_base::sync_with_stdio(false); cin.tie(0);
    cin >> n >> d;
    for(int i = 1;i <= n;i++) {
        cin >> a[i];
    }
    ll mx = -llINF, mn = llINF;
    for(int i = 1;i <= n;i++) {
        mx = max(mx,a[i]+d*i), mn = min(mn,a[i]+d*i);
    }
    ll ans = (mx-mn);
    mx = -llINF, mn = llINF;
    for(int i = 1;i <= n;i++) {
        mx = max(mx,a[i]-d*i), mn = min(mn,a[i]-d*i);
    }
    ans = min(ans,(mx-mn));
    if(ans % 2LL == 0) cout << ans/2 << ".0";
    else cout << ans/2 << ".5";
}