"use strict"

const arg_str = require('fs').readFileSync('/dev/stdin', 'UTF-8')
const args = arg_str.split('\n')
const N = +args[0]
const cities = args.slice(1).map(x => x.split(' '))

let sum = 0

for(let i=0;i<N;i++){
  for(let j=0;j<N;j++){
    if(i != j){
      sum += Math.sqrt((cities[i][0] - cities[j][0]) * (cities[i][0] - cities[j][0]) + (cities[i][1] - cities[j][1]) * (cities[i][1] - cities[j][1]))
    }
  }
}

const fact = k => 
  k===0 ?  1
  : k * fact( k - 1 )

console.log(sum/fact(N))
