#include <iostream>
#include <fstream>

using namespace std;

int main(){
	ifstream fin("A.in");
	int n, w, h[11];
	while(fin >> n >> w, n){
		memset(h, 0, sizeof(h));
		int mx = 0, tmp;
		for(int i=0;i<n;i++){
			fin >> tmp;
			mx = max(mx, tmp);
			h[tmp/w]++;
		}
		int size = mx/w, hist = 0;
		for(int i=0;i<=size;i++)
			hist = max(hist, h[i]);
		double ans = 0.01;
		for(int i=0;i<size;i++)
			ans += ((double)(size-i)*h[i])/(hist*size);
		printf("%.6lf\n", ans);
	}
}