#include<cstdio>
using namespace std;
FILE *f=fopen("dezastru.in","r");
FILE *g=fopen("dezastru.out","w");
int n, k, st[30];
float v[30], s, nr=1;
long long numar=1;
int valid(int p)
{
    int i;
    for(i=1;i<p;i++)if(st[i]==st[p])return 0;
   // if(p<=k)nr*=v[st[p]];
    return 1;
}
void fact()
{
    int i;
    for(i=1;i<=n;i++)numar*=i;
}
int solutie(int p)
{
    if(p==n)return 1;
    return 0;
}
void bkt(int p)
{
    int i;
    for(i=1;i<=n;i++)
    {
        st[p]=i;
        if(valid(p))
        {
            if(solutie(p))
            {
                int j;
                for(j=1;j<=k;j++)nr*=v[st[j]];
                s+=(float)(nr/numar);
                nr=1;
            }
            else bkt(p+1);
        }
    }
}
int main()
{
    fscanf(f,"%d %d",&n,&k);
    int i;
    fact();
    for(i=1;i<=n;i++)fscanf(f,"%f",&v[i]);
    bkt(1);
    fprintf(g,"%6f",s);
    fclose(f);
    fclose(g);
    return 0;
}