#include <fstream>

using namespace std;
ifstream cin("dezastru.in");
ofstream cout("dezastru.out");
double C[30],rez,r,v[30];
long long n,k,nr,nk_fact,kfact;
void comb(int x);
int main()
{
    cin>>n>>k;
    nk_fact=kfact=1;
    for(int i=n;i>n-k;i--)
            nk_fact*=i;
    for(int i=2;i<=k;i++)
        kfact*=i;
    for(int i=1;i<=n;i++)
        cin>>v[i];
    C[0]=1;nr=0;
    comb(0);
    cout<<rez<<'\n';
    return 0;
}
void comb(int x)
{
    if(nr==k)
    {
        r=C[0];
        r=r*kfact;
        r=r/nk_fact;
        rez+=r;
    }
    else if(x+1<=n){
            C[0]=C[0]*v[x+1];nr++;
            C[x+1]=1;
            comb(x+1);
            C[0]=C[0]/v[x+1];nr--;
            C[x+1]=0;
            comb(x+1);

    }
}