#include <fstream>
#include <iomanip>
#define FOR(a,b,c) for(int a=b; a<=c; ++a)
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int c[30][30], n, k;
float x[30], y[30][30];
int main(){
    f>>n>>k;
    FOR(i,1,n)
        f>>x[i];
    y[0][0]=c[0][0]=1;
    FOR(i,1,n)
    {
        y[i][0]=c[i][0]=1;
        FOR(j,1,k)
        {
            y[i][j]=y[i-1][j]+y[i-1][j-1]*x[i];
            c[i][j]=c[i-1][j]+c[i-1][j-1];
        }
    }
    g<<setprecision(6)<<fixed;
    g<<y[n][k]/c[n][k]<<"\n";
   return 0;
}