#include<fstream>
#include<iomanip>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
double v[26],a[26][26];

int main()
{
    int n,k,i,j,x=1;
    double sol;
    f>>n>>k;
    for(i=1;i<=n;i++)
        f>>v[i];
    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]+a[i-1][j-1]*v[i];
    }
    for(i=k+1;i<=n;i++)
        x*=i,x/=i-k;
    sol=(double)a[n][k]/x;
    g<<fixed<<setprecision(6)<<sol;
    return 0;
}