#include<fstream.h>
#include<iomanip.h>
long double v[26],mat[26][26],rez;
unsigned long comb[26][26];
int main()
{ifstream f("dezastru.in");
ofstream g("dezastru.out");
int n,k,i,j;
f>>n>>k;
for(i=1;i<=n;i++) f>>v[i];
f.close();
comb[1][0]=comb[1][1]=1;
mat[1][1]=v[1];mat[1][0]=1;
for(i=2;i<=n;i++)
 {comb[i][0]=1;
  mat[i][0]=1;
  for(j=1;(j<=i)&&(j<=k);j++)
   {mat[i][j]=mat[i-1][j]+mat[i-1][j-1]*v[i];
    comb[i][j]=comb[i-1][j]+comb[i-1][j-1];
   }
 }
rez=mat[n][k]/comb[n][k];
//cout<<mat[n][k]<<" "<<comb[n][k]<<'\n';
g<<setprecision(6)<<rez<<'\n';
g.close();
return 0;
}