function myin(){return require("fs").readFileSync("/dev/stdin", "utf8").trim();}
function myout(t){console.log(t);}//standard output
//[no]param
//0:何もしない  1:数値へ変換  2:半角SPで分割  3:改行で分割  4:半角SPで分割し、数値配列へ
//5:改行で分割し、数値配列へ  6:1文字に分割  7:1文字に分割し、数値配列へ
function myconv(i,no){switch(no){case 0:return i;case 1:return parseInt(i);case 2:return i.split(" ");case 3:return i.split("\n");case 4:return i.split(" ").map((a)=>Number(a));case 5:return i.split("\n").map((a)=>Number(a));case 6:return i.split("");case 7:return i.split("").map((a)=>Number(a));}}

function Main(input) {
  input = myconv(input,3);
  var N = myconv(input[0],1);
  var xList = new Array(N);
  var yList = new Array(N);
  input.shift();
  for(var i = 0; i < N; i++){
    var tmp = myconv(input[i],4);
    xList[i] = tmp[0];
    yList[i] = tmp[1];
  }
  var sum = 0;
  for(var i = 0; i < N-1; i++){
    for(var j = i+1; j < N; j++){
      var x = Math.abs(xList[i] - xList[j]);
      var y = Math.abs(yList[i] - yList[j]);
      //myout("i:" + i + ",j:" + j + ",x" + x+",y:"+y);
      sum += Math.sqrt(Math.pow(x,2) + Math.pow(y,2));
    }
  }
  //myout(xList);
  //myout(yList);
  sum *= 2;
  var fact = 1;
  for(var i = 1; i <= N; i++){
    fact *= i;
  }
  //myout(sum / fact);
  myout(sum / N);
}

Main(myin());
