#include <cstdio>
using namespace std;
float a[30],sol;
int n,k,np;
void comb(int pz,int nm,float p)
 {
 if(nm==k+1)
  {
  sol+=p;
  ++np;
  }
 else
 for(int i=pz+1;i<=n-(k-nm);i++)
  comb(i,nm+1,p*a[i]);
 }
int main()
 {
 FILE *f=fopen("dezastru.in","r");
 FILE *g=fopen("dezastru.out","w");
 fscanf(f,"%d %d",&n,&k);
 int i;
 for(int i=1;i<=n;i++) fscanf(f,"%f",&a[i]);
 comb(0,1,1.00);
 sol/=np;
 fprintf(g,"%.6f",sol);
 return 0;
 }