#include<stdio.h>
int n,a[26],ex[26],m,i;
long long unsigned p;
float sum,ab,exe[26];

void back(int i)
{int k,x;

     for(k=1;k<=n;k++)
     {a[i]=k;
                      if(!ex[k])
     {
                      
                      ex[k]=1;
                      if(i==m)
                      {      for(x=1;x<=m;x++)
                             ab*=exe[a[x]];
                             sum+=ab/p;
                             ab=1;
                             ex[k]=0;
                          }
                              else
                              {back(i+1);
                              ex[k]=0;
                              }
                              }
                              }
                              }
int main ()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d%d",&n,&m);
    for(i=1;i<=n;i++)
    scanf("%f",&exe[i]);
 p=1;
 for(i=1;i<=n;i++)
   p*=i;
   ab=1;
   back(1);
   printf("%.6llf",sum);
    return 0;
}