"use strict";
function calc(arrayX, arrayY, curX, curY) {
  if (arrayX.length === 0) return 0;
  let sum = 0;
  // ないとき
  if (typeof curX === "undefined") {
    for (let curIndex = 0; curIndex < arrayX.length; curIndex++) {
      const copyArrayX = arrayX.concat([]);
      const copyArrayY = arrayY.concat([]);
      const x = copyArrayX.splice(curIndex, 1);
      const y = copyArrayY.splice(curIndex, 1);
      sum += calc(copyArrayX, copyArrayY, x, y);
    }
    return sum;
  }

  // curXあるとき
  for (let nextIndex = 0; nextIndex < arrayX.length; nextIndex++) {
    sum += Math.sqrt(
      (curX - arrayX[nextIndex]) * (curX - arrayX[nextIndex]) +
        (curY - arrayY[nextIndex]) * (curY - arrayY[nextIndex])
    );
    const copyArrayX = arrayX.concat([]);
    const copyArrayY = arrayY.concat([]);
    const x = copyArrayX.splice(nextIndex, 1);
    const y = copyArrayY.splice(nextIndex, 1);
    sum += calc(copyArrayX, copyArrayY, arrayX[nextIndex], arrayY[nextIndex]);
  }
  return sum;
}

function Main(inputs) {
  // 1行目がinput[0], 2行目がinput[1], …に入る
  const input = inputs.split("\n");
  const n = Number(input[0]);
  const x = [];
  const y = [];
  for (let i = 0; i < n; i++) {
    x.push(input[i + 1].split(" ")[0]);
    y.push(input[i + 1].split(" ")[1]);
  }
  const kaijo = [0, 1, 2, 6, 24, 120, 720, 5040, 40320];
  console.log(calc(x, y) / kaijo[n]);
}
//*この行以降は編集しないでください（標準入出力から一度に読み込み、Mainを呼び出します）
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
