#include <iostream>
#include <cstdio>
using namespace std;
float a[100],s=1,sol;
int numi,k,n,fr[100],kk;
FILE *f=fopen ("dezastru.in","r");
FILE *g=fopen ("dezastru.out","w");
void Back (int k)
{
    int i,j;
    for (i=1;i<=n;i++)
    {
        fr[i]++;
        if (fr[i]==1)
        {
            s*=a[i];
        if (k<kk) {Back(k+1);} else
         {sol+=s; }
            s/=a[i];
        }

        fr[i]--;
    }
}


int main()
{
    int i;
fscanf (f,"%d %d",&n,&kk);
numi=1;
for (i=1;i<=n;i++) {
    fscanf (f,"%f",&a[i]);
    numi*=i;
}
Back (1);

//g<<fixed<<setprecision(7)<<(double)sol/numi;
float rezol=sol/numi;
fprintf (g,"%.6f",rezol);

    return 0;
}