#include<fstream>
#include<iomanip>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
float a[26][25],v[26],valf;
int c[26][26],n,k,i,j;
int main(){
   fin>>n>>k;
   for (i=1;i<=n;i++)
    fin>>v[i];
   fin.close();
   a[0][0]=1;
   for (i=1;i<=n;i++){
      a[i][0]=1;
      for (j=1;j<=i;j++)
        a[i][j]=a[i-1][j-1]*v[i]+a[i-1][j];
   }
   c[0][0]=1;
   for (i=1;i<=n;i++){
     c[i][0]=1;
     for (j=1;j<=i;j++)
        c[i][j]=c[i-1][j-1]+c[i-1][j];
   }
   valf=a[n][k]/(float)c[n][k];
   fout<<fixed<<setprecision(6)<<valf;
   fout.close();
   return 0;
}