#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int n, k, comb[28][28], i, j;
double val[28], d[28][28];
int main()
{
    f>>n>>k;
    for (i=1; i<=n; i++)
        f>>val[i];
    for (i=0; i<=n; i++) {
        comb[i][0]=1;
        d[i][0]=1;
    }
    for (i=1; i<=n; i++) {
        for (j=1; j<=k; j++) {
            d[i][j]=d[i-1][j]+d[i-1][j-1]*val[i];
            comb[i][j]=comb[i-1][j]+comb[i-1][j-1];
        }
    }
    g<<fixed<<setprecision(6)<<(float)d[n][k]/comb[n][k];
    return 0;
}