#include <fstream>
#include <iomanip>
#define N 30

using namespace std;

ifstream f("dezastru.in");
ofstream g("dezastru.out");

int n,i,j,k;
double D[N][N],v[N],c=1;

int main() {
    f >> n >> k;
    for (i=1;i<=n;D[i][0]=1.0,i++) f >> v[i];
    D[0][0]=1.0;
    for (i=1;i<=n;i++)
        for (j=1;j<=min(i,k);j++)
            D[i][j]=D[i-1][j]+D[i-1][j-1]*v[i];
    for (i=k+1;i<=n;i++) c*=i;
    for (i=1;i<=n-k;i++) c/=i;
    D[n][k]/=(double)c;
    g << fixed;
    g << setprecision(6) << D[n][k] << '\n';
    f.close();g.close();
    return 0;
}