#include <bits/stdc++.h>
using namespace std;

#ifdef DEBUG
#define fast
#else
#define fast cin.tie(0)->sync_with_stdio(0)
#define endl '\n'
#define cerr if(1);else cerr
#endif
#define _ <<' '<<
#define ALL(v) v.begin(),v.end()
#define ft first
#define sd second

using ll = long long;
using ld = long double;
using pii = pair<int,int>;

signed main() {
    fast;

    ll n,d;
    cin>>n>>d;
    ll l, r;
    l=1e18, r=-1e18;
    for(ll i=0;i<n;i++)
    {
        ll a;
        cin>>a;
        a-=i*d;
        l=min(l, a);
        r=max(r, a);
    }
    cout<<(r-l)/2<<'.'<<(r-l)%2*5<<'\n';

    cerr _ "meow" _ endl;

    return 0;
}