#include <iostream>
#include <vector>
using namespace std;
using ld = long double;
using ll = long long;

vector<ll> vals;
int main() {
	int n, d; cin >> n >> d;
	vals.resize(n);

	ll lo = 1e18, hi = -1e18;
	for (int i = 0; i < n; i++) {
		cin >> vals[i];
		lo = min(lo, vals[i]-i*d);
		hi = max(hi, vals[i]-i*d);
	}

	ll val = hi-lo;
	if (val%2) {
		cout << val/2 << ".5" << endl;
	}
	else cout << val/2 << ".0" << endl;
}