"use strict";

const main = arg => {
  const n = parseInt(arg.split("\n")[0], 10)
  const xy = arg.split("\n").slice(1).map(e => e.split(" "))
  let total = 0
  for (let i = 0; i < n; i++) {
    for (let j = i + 1; j < n; j++) {
		const diffx = xy[j][0] - xy[i][0]
		const diffy = xy[j][1] - xy[i][1]
        const r = Math.sqrt(diffx * diffx + diffy * diffy)
        total += r
    }
  }
  console.log(total * 2 / n)
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));