// dfmt off
T lread(T=long)(){return readln.chomp.to!T;}T[] lreads(T=long)(long n){return iota(n).map!((_)=>lread!T).array;}
T[] aryread(T=long)(){return readln.split.to!(T[]);}void arywrite(T)(T a){a.map!text.join(' ').writeln;}
void scan(L...)(ref L A){auto l=readln.split;foreach(i,T;L){A[i]=l[i].to!T;}}alias sread=()=>readln.chomp();
void dprint(L...)(lazy L A){debug{auto l=new string[](L.length);static foreach(i,a;A)l[i]=a.text;arywrite(l);}}
static immutable MOD=10^^9+7;alias PQueue(T,alias l="b<a")=BinaryHeap!(Array!T,l);import std;
// dfmt on

void main()
{
    long N = lread();
    auto X = new real[](N);
    auto Y = new real[](N);
    foreach (i; 0 .. N)
    {
        scan(X[i], Y[i]);
    }
    auto d = new real[][](N, N);
    foreach (i; 0 .. N)
        foreach (j; i .. N)
        {
            d[i][j] = d[j][i] = sqrt((X[i] - X[j]) ^^ 2 + (Y[i] - Y[j]) ^^ 2);
        }
    auto route = iota(N).array;
    real s = 0;
    long c;
    dprint(d);
    do
    {
        c++;
        real tmp = 0;
        foreach (i; 0 .. N - 1)
        {
            tmp += d[route[i]][route[i + 1]];
        }
        dprint(route, tmp);
        s += tmp;
    }
    while (nextPermutation(route));
    writefln("%.10f", s / c);
}
