#include <bits/stdc++.h>

using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
vector<double> a;
const double z=0.0000001;
const int x=10000000;
double n,k,s;
int sol;
long long fac(long long);

int main()
{
    f>>n>>k;
    for(int i=1;i<=n;i++)
        f>>a[i];
     do
    {
        s+=(a[1]*a[2])/fac(n);
    }
    while(next_permutation(a.begin(),a.end()));
    sol=(int)s*x;
    g<<fixed<<setprecision(6);
    g<<s-sol%10*z;
    return 0;
}
long long fac(long long n)
{
    long long p=1;
    for(int i=2;i<=n;i++)
        p*=i;
    return p;
}