#include <cstdio>
#include <algorithm>

using namespace std;

int a[10],n,i,k,p;
float v[1001],b[30],s;

int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d%d",&n,&k);
    for(i=1;i<=n;i++)
    {
        scanf("%f",&b[i]);
        a[i]=i;
    }
    do{
        p++;v[p]=1;
        for(i=1;i<=k;i++)v[p]*=b[a[i]];
    }while(next_permutation(a+1,a+n+1));
    for(i=1;i<=p;i++)s=s+v[i]/float(p);
    printf("%.6f",s);
    return 0;
}