//And He found you lost and guided [you] [93:07]

//#pragma GCC optimize ("Ofast")
//#pragma GCC target ("avx2")
//#pragma GCC optimize("unroll-loops")

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

typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> pll;
typedef pair<int,int> pii;
typedef vector<ll> vll;
typedef vector<int> vii;
typedef map<int,int> mpi;
typedef map<ll,ll> mpl;
typedef unordered_map<int,int> umpi;
typedef unordered_map<ll,ll> umpl;

#define modu 998244353 
#define mod 1000000007
#define eps 1e-7
#define inf 1000000000000000006
#define ff first
#define ss second
#define pb push_back
#define all(v) v.begin(), v.end()
#define fastio ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define endl '\n'
#define pi acos(-1.0)
#define dec(n) fixed << setprecision(n)
#define N 300005
#define int __int128

int n,d,a[N];
int c[N];
int f(int sft){
    
    c[1]=a[1];
    int f1=1,del1=0;
    c[1]-=sft;
    for(int i=2;i<=n;i++){
        c[i]=c[i-1]+d;
        if(c[i]>a[i]+sft) f1=0;
        int gap=max((int)0,a[i]-sft-c[i]);
        c[i]=c[i]+gap;
        del1+=gap; 
        // cout << gap << " ";

    }
    if(del1>2*sft) f1=0;
    c[1]=a[1];
    c[1]-=sft;
    for(int i=2;i<=n;i++) c[i]=c[i-1]+d;
    for(int i=1;i<=n;i++){
        if(del1+c[i]>a[i]+sft) f1=0;
    }

    c[1]=a[1];
    int f2=1,del2=0;
    c[1]+=sft;
    for(int i=2;i<=n;i++){
        c[i]=c[i-1]-d;
        if(c[i]<a[i]-sft) f2=0;
        int gap=max((int)0,c[i]-a[i]-sft);
        c[i]=c[i]-gap;
        del2+=gap; 

    }
    c[1]=a[1];
    c[1]+=sft;
    for(int i=2;i<=n;i++) c[i]=c[i-1]-d;

    for(int i=1;i<=n;i++){
        if(c[i]-del2<a[i]-sft) f2=0;
    }

    if(del2>2*sft) f2=0;


    return (f1 || f2);

}


long long NN,D,A[N];

void solve(){
    cin >> NN >> D;
    n=NN;
    d=D;
    for(int i=1;i<=n;i++){
        cin >> A[i];
        a[i]=A[i]*2LL;
    } 

    d*=2;


    int lo=0,hi=1e25+1,ans;
    while(lo<=hi){
        int mid=(lo+hi)/2;
        if(f(mid)){
            ans=mid;
            hi=mid-1;
        }
        else lo=mid+1;
    }



    cout << (long long)ans/2LL << ".";
    if(ans%2) cout << 5 << endl;
    else cout << 0 << endl;

    
}


signed main(){
    fastio;
    //srand(chrono::steady_clqk::now().time_since_epqh().count());
    // fflush(stdout);
    int T=1,cs=0;
    // cin >> T;
    while(T--){
        //cout << "Case " << ++cs << ": " ; 
        solve();
    } 
    return 0;
}