#include <fstream>
#include <iostream>
#include <iomanip>
using namespace std;
int b[30][30], n, k, i, j;
double a[30][30], v[50], nr;
int main()
{
    ifstream cin("dezastru.in");
    ofstream cout("dezastru.out");
    cin>>n>>k;
    for (i=1; i<=n; i++)
        cin>>v[i];
    for (i=0; i<=n; i++)
    {
        a[i][0]=1;
        b[i][0]=1;
    }
    for (i=1; i<=n; i++)
        for (j=1; j<=k; j++)
        {
            a[i][j]=(a[i-1][j]+a[i-1][j-1]*v[i]);
            b[i][j]=b[i-1][j]+b[i-1][j-1];
        }
    nr=a[n][k]/(double)b[n][k];
    cout<<nr<<setprecision(6)<<fixed;
}