#include<iostream.h>
#include<stdio.h>
FILE *f=fopen("dezastru.in","r"),*g=fopen("dezastru.out","w");
float a[30];double aux,rez;
int ut[30],st[30],n,t,nr;
void back(int k)
{
    for(int pval=1;pval<=n;pval++)
    {
        st[k]=pval;
        if(!ut[pval]&&k<=n)
        {
            ut[pval]=1;
            if(k==t)
            {
                aux=1;nr++;
		for(int i=1;i<=t;i++)
                    aux*=a[st[i]];
                rez+=aux;
                ut[pval]=0;
            }
            else
                back(k+1),ut[pval]=0;
        }
    }
}
int main()
{
    fscanf(f,"%d%d",&n,&t);
    for(int i=1;i<=n;i++)
        fscanf(f,"%f",&a[i]);
    back(1);
    fprintf(g,"%6lf",rez/nr);
    return 0;
}