#include <cstdio>
#include <fstream>

using namespace std;

long int i,j,n,k,sol[30];
long long int fact;
float a[30],s1,s;

long int ok(int x)
{
    int i;
    for (i=1;i<=x-1;i++)
        if (sol[i]==sol[x]) return 0;
        return 1;
}

void back(int x)
{
    int i,j;
    if (x>k) {
        s1=1;
        for (j=1;j<=k;j++)
        s1=s1*a[sol[j]];
        s=s+s1;
    }
    else
    for (i=1;i<=n;i++) {
        sol[x]=i;
        if (ok(x)) back(x+1);
    }
}

int main()
{
    ifstream f("dezastru.in");
    freopen("dezastru.out","w",stdout);
    f>>n>>k;
    for (i=1;i<=n;i++)
    f>>a[i];
    fact=1;
    for (i=2;i<=n;i++)
    fact=fact*i;

    back(1);
    printf("%0.6f",s/fact);

    f.close();
    fclose(stdout);
    return 0;
}