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