#include<bits/stdc++.h>
using namespace std;
#define int long long
const long double eps = 1e-9;
const int N = 1000006;
int ara[N],rev[N];
int n,d;
bool pos(long double a,int ara[]){
    long double l0 = ara[0]-a;
    long double r0 = ara[0]+a;

    for(int i=1;i<n;i++){

        long double l2 = ara[i]-a;
        long double r2 = ara[i]+a;

        l2 = max(l2,l0+d);
        r2 = min(r2,r0+d);
        if( l2-r2>eps ) return false;
        l0 = l2;
        r0 = r2;
    }
    return true;
}

signed main(){
    long double l = 0;
    long double r = 1e18;
    cin>>n>>d;
    for(int i=0;i<n;i++) {
        cin>>ara[i];
        rev[n-i-1] = ara[i];
    }

    for(int i=0;i<200;i++){
        long double mid = (l+r)/2.;
        if(pos(mid,ara) or pos(mid,rev)) r = mid;
        else l = mid;
    }
    cout<<fixed<<setprecision(1)<<r<<endl;
}