1 //Auther: Madhat Alsoos
    2 
    3 #include <fstream>
    4 #include <iostream>
    5 
    6 using namespace std;
    7 
    8 int bar[200];
    9 int main()
   10 {
   11   //freopen("c.in","r",stdin);
   12   int n,w;
   13   while (cin>>n>>w)
   14     {
   15       if (n==0 && w==0) return 0;
   16       int i;
   17       for (i=0;i<190;i++)
   18         bar[i]=0;
   19       for (i=0;i<n;i++)
   20         {
   21           int a;
   22           cin>>a;
   23           bar[a/w]++;
   24         }
   25       int maxx=0;
   26       int lastindex=-1;
   27       for (i=0;i<150;i++)
   28         {
   29           if (bar[i]>maxx)
   30             maxx=bar[i];
   31           if (bar[i]>0) lastindex=i;
   32         }
   33       double ans=0.01;
   34       for (i=0;i<=lastindex;i++)
   35         ans+=(bar[i]/(double)maxx)*(lastindex-i)/lastindex;
   36       printf("%4.6f\n",ans);
   37     }
   38   return 0;
   39 }