'use strict';
const main = input => {
  const n = parseInt(input[0]);
  let p = [];
  for(let i=0; i<n; i++) {
    let k = input[i+1].split(' ').map(val => parseInt(val));
    p[i] = {x: k[0], y: k[1]};
  }
  let distanceSum = 0;
  for(let i=0; i<n-1; i++) {
    for(let j=i; j<n; j++) {
      distanceSum += Math.sqrt(Math.pow(p[i].x-p[j].x, 2)+Math.pow(p[i].y-p[j].y, 2));
    }
  }
  console.log(distanceSum/n*2);
}
main(require("fs").readFileSync("/dev/stdin", "utf8").split('\n'));