#include<bits/stdc++.h>
#define int long long
using namespace std;
int const N = 1e6 + 5;
int a[N];
int n;
bool check(int k, int d){
    int l = a[0] - k, r = a[0] + k;
    for(int i = 1; i < n; i++){
        l = max(l, a[i] - k - d * i);
        r = min(r, a[i] + k - d * i);
        if(r < l)
            return 0;
    }
    if(r < l)
        return 0;
    return 1;
}
signed main(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    int d;
    cin >> n >> d;
    d *= 2;
    for(int i = 0; i < n; i++){
        cin >> a[i];
        a[i] *= 2;
    }
    int minn = 1e18;
    int l = 0, r = 2e17;
    while(l < r){
        int mid = l + r >> 1;
        if(check(mid, d)){
            r = mid;
        }else{
            l = mid + 1;
        }
    }
    minn = min(minn, l);
    l = 0, r = 2e17;
    while(l < r){
        int mid = l + r >> 1;
        if(check(mid, -d)){
            r = mid;
        }else{
            l = mid + 1;
        }
    }
    minn = min(minn, l);
    cout << minn / 2;
    if(minn & 1){
        cout << ".5\n";
    }else{
        cout << ".0\n";
    }
}