#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;

    int n;
    ll d;
    cin >> n >> d;
    vector<ll> v(n);
    for (auto &x: v) cin >> x;

    d *= 100;
    for (auto &x: v) x *= 100;

    auto test = [&](ll m) {
        ll l = v[0] - m, r = v[0] + m;
        for (int i = 1; i < n; i++) {
            l = max(l+d, v[i]-m);
            r = min(r+d, v[i]+m);
            if (l > r)
                return false;
        }
        return l <= r;
    };

    ll l = 0, r = 3e16 * 100;
    while (l < r) {
        ll m = (l+r) / 2;
        if (test(m))
            r = m;
        else
            l = m+1;
    }

    int x = l % 10;
    if (x < 5)
        l -= x;
    else
        l += 10 - x;

    cout << l / 100 <<'.'<< l/10%10 << endl;

    return 0;
}