#include<bits/stdc++.h>
#define endl '\n'
using namespace std;
typedef long long ll;
const ll inf = 1e6;
void read()
{
    freopen("triangles.in","r",stdin);
    freopen("triangles.out","w",stdout);
}
long long p;
double inp;
long double k,q;
vector <long long> v;
int i,j,n;
long double ans;
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    read();
    cin>>n;
    string s;
    for(i=1; i<=n; i++)
    {
        cin >> s;
        double inp = stod(s);
        p = (ll)(inp*inf);
        v.push_back(p);
    }
    sort(v.begin(),v.end());

        j = n-1;
        int st = (n%2);
        for(i=st; i<j; i++)
        {
            q = v[i]/inf;
            p = v[i]%inf;
            q = q+p*1.0/inf;
            k = v[j]/inf;
            p = v[j]%inf;
            k = k+p*1.0/inf;
            ans+=sqrt(q*q+k*k);
            j--;
        }


    cout<<fixed<<setprecision(9)<<ans<<endl;
    return 0;
}