#include<fstream.h>

int n,k,t,u,x[40],i;

long double p,s,a[40];

int main()
{
 ifstream f("dezastru.in");
 ofstream g("dezastru.out");
 f>>n>>t;
 for(i=1;i<=n;i++)f>>a[i];
 k++;
 s=0;
 while(k)
 {
  x[k]++;
  if(k==t)
   {
    p=1;
    for(i=1;i<=t;i++)p*=a[x[i]];
    s+=p;
    u=0;
    while(x[k-u]==n-u)u++;
    k-=u;
    }
  else
   {
    k++;
    x[k]=x[k-1];
    }
  }
 for(i=t+1;i<=n;i++)s*=((n+1-t)/t);
 g<<s;
 g.close();
 return 0;
 }