#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int n,k,i,j,c[26][26];
double a[26],m[26][26],l;
int main()
{
    f>>n>>k;
    for (i=1;i<=n;i++)
         f>>a[i];
    for (i=0;i<=n;i++)
    c[i][0]=m[i][0]=1;
    for (i=1;i<=n;i++)
         for (j=1;j<=k;j++)
         m[i][j]=m[i-1][j-1]*a[i]+m[i-1][j];
    for (i=1;i<=n;i++)
         for (j=1;j<=k;j++)
          c[i][j]=c[i-1][j]+c[i-1][j-1];
    l=double(m[n][k])/double(c[n][k]);
    g<<fixed<<setprecision(6)<<l;
    return 0;
}