#include<cstdio>     

int N,K,viz[30],nrd;
double a[30],s,p;
  
void citire()
{
  scanf("%d %d",&N,&K);
  for(int i=0;i<N;i++)
    scanf("%lf",&a[i]);
}


void back(int k,int poz,double s)
{
  if(k==K)
  {
    p+=s;
    nrd++;
    return;
  }
  for(int i=poz;i<N-K+k+1;++i)
      back(k+1,i+1,s*a[i]);
    
   
}

void solve()
{
  p=0;
  back(0,0,1);
  printf("%.6lf",p/nrd);
} 
  
int main()
{
  freopen("dezastru.in","r",stdin);
  freopen("dezastru.out","w",stdout);
  citire();
  solve();
  return 0;
}