function Main(input) {
  var lines = input.split("\n").filter(v=>v!=='')
  var body = lines.slice(1)

  var n = +lines.slice(0,1)
  var poss = body.map(s=>s.split(' ').map(v=>+v))
  var cache = {}
  var sum = 0

  let diff = (x1,y1,x2,y2) => {
    var v = Math.sqrt(((x1-x2)**2) + ((y1-y2)**2))
    console.log({x1,y1,x2,y2,v})
    return v
  }

  let f = (past_poss,use_poss) => {
    if(use_poss.length == 0) {
      let d = 0;
      for(var i=1;i<past_poss.length;i++){
        var x = +past_poss[i-1][0]
        var y = +past_poss[i-1][1]
        var _x = +past_poss[i][0]
        var _y = +past_poss[i][1]
        sum += diff(x,y,_x,_y)
      }
      console.log({sum})
      return
    }
    use_poss.forEach(_pos=>{
      //sum += diff(x,y,_x,_y)
      f([...past_poss,_pos],use_poss.filter(__pos=>__pos.toString() != _pos.toString()))
    })
  }
  poss.forEach(_pos=>{
    f([_pos],poss.filter(__pos=>__pos.toString() != _pos.toString()))
  })
  var b = 1
  for(;n>0;n--){
    b *=  n
  }
  console.log(sum/b)
}
//*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"));
