#include<stdio.h>    
   int main()    
    {int k=1,up,i,m,n,s[30];     
	 float v[30],q=0,w;
     freopen("dezastru.in","r",stdin);    
     freopen("dezastru.out","w",stdout);    
     scanf("%d%d",&n,&m);   
	 for (i=1;i<=n;i++) scanf("%f",&v[i]);
     for (s[k]=0;k;)    
        {    
        up=0;    
       while(!up&&s[k]<n-m+k)    
            {    
             s[k]++;    
             up=1;    
             }    
         if(up)    
             if(k==m)  
                 {   w=1;
			         for(i=1;i<=m;i++)  w*=v[s[i]];  
                     q+=w*2; 
                     					 
                  }       
             else k++,s[k]=s[k-1];    
         else    
             k--;    
         } 
    for (i=2;i<=n;i++) q/=i; 
         printf("%.6lf",q);
     return 0;    
     }