import std.stdio, std.conv, std.string, std.array, std.range, std.algorithm, std.container;
import std.math, std.random, std.bigint, std.datetime, std.format;
void main(string[] args){ if(args.length > 1) if(args[1] == "-debug") DEBUG = 1; solve(); }
void log()(){ writeln(""); } void log(T, A ...)(T t, lazy A a){ if(DEBUG) write(t, " "), log(a); } bool DEBUG = 0; 
string rstring(){ static string[] ss; while(!ss.length) ss = readln.chomp.split; string res = ss[0]; ss.popFront; return res; }
T rtype(T)(){ return rstring.to!T; } alias rint = rtype!int, rlong = rtype!long, rreal = rtype!real;
T[] rtypes(T)(int n){ return n.iota.map!(i => rtype!T()).array; } alias rint = rtypes!int, rlong = rtypes!long, rreal = rtypes!real;
T[][] rtypess(T)(int n, int m){ return n.iota.map!(i => rtypes!T(m)).array; } alias rint = rtypess!int, rlong = rtypess!long, rreal = rtypess!real;

// ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- //

void solve(){
	int n = rint;
	
	P[] ps;
	foreach(i; 0 .. n) ps ~= P(rreal, rreal);
	
	real sum = 0.0;
	foreach(p; ps) foreach(q; ps){
		P d = P(p.x - q.x, p.y - q.y);
		sum += sqrt(d.x * d.x + d.y * d.y);
	}
	
	real ans = sum / n;
	
	writefln("%10.10f", ans);
	
}

struct P{
	real x, y;
}

/*
全部でN!×(N-1)個の通路。
通路の種類はN(N-1)/2種類ですべて同回数使われるから
平均は Sum * 2 / N(N-1) それが(N-1)個
*/
