#include <iostream>
#include <fstream>
using namespace std;
ifstream in ("dezastru.in");
ofstream out("dezastru.out");
int const N=26;
int n,k,total,c[2][N];
float suma,a[N][N],v[N];
bool marcat[N];
void combinari()
{
    //for(int i=0;i<=n;i++)
        //c[i][0]=1;
    c[0][0]=c[1][0]=1;
    for(int i=0;i<=n;i++)
        for(int j=0;j<n;j++)
            c[(i+1)%2][j+1]=c[i%2][j]+c[i%2][j+1];
}
int main()
{
    in>>n>>k;
    for(int i=1;i<=n;i++)
        in>>v[i];
    for(int i=0;i<=n;i++)
        a[i][0]=1;
    a[1][1]=v[1];
    for(int i=2;i<=n;i++)
        for(int j=1;j<=i;j++)
            a[i][j]=a[i-1][j]+a[i-1][j-1]*v[i];
    combinari();
    out<<a[n][k]/c[n%2][k]<<"\n";
    return 0;
}