void main() {
    int n = readln.chomp.to!int;
    int[] x = new int[n], y = new int[n];
    double s = 0.0;
    foreach (i; 0 .. n) {
        int[] tmp = readln.split.to!(int[]);
        x[i] = tmp[0], y[i] = tmp[1];
    }
    foreach (i; 0 .. n) {
        foreach (j; 0 .. n) {
            if (i == j) continue;
            int xdiff = x[i] - x[j];
            int ydiff = y[i] - y[j];
            s += sqrt(xdiff.to!double^^2+ydiff.to!double^^2);
        }
    }
    writefln("%.10f", s / n);
}

import std.stdio;
import std.string;
import std.array;
import std.conv;
import std.algorithm;
import std.range;
import std.math;
import std.numeric;
import std.container;
import std.typecons;
import std.ascii;
import std.uni;