module Main where

import Control.Monad

type Point = (Integer, Integer)
type Polygon = [Point]
type Side = (Point, Point)
type PolygonSides = [Side]

indexes :: [a] -> [Int]
indexes x =
    [0..l - 1] where l = length x

sibling :: [a] -> Int -> (a, a)
sibling l i = 
    if i == (length l) - 1
        then (l !! i, l !! 0)
        else (l !! i, l !! (i + 1))

parsePolygon :: String -> Polygon
parsePolygon s =
    map parsePoint $ lines s
    where 
        parsePoint :: String -> Point
        parsePoint s = (list !! 0, list !! 1) where list = map read $ words s

sideLength :: Side -> Float
sideLength ((x1, y1), (x2, y2)) =
    sqrt $ (x1' - x2') ** 2 + (y1' - y2') ** 2
    where [x1', y1', x2', y2'] = map fromIntegral [x1, y1, x2, y2]

perimeter :: Polygon -> Float
perimeter polygon =
    sum $ map sideLength sides
    where
        polygonSides :: Polygon -> PolygonSides
        polygonSides p = map (sibling polygon) (indexes p)
        sides = polygonSides polygon

main :: IO()
main = do
    _ <- readLn :: IO Int
    contents <- getContents
    print $ perimeter $ parsePolygon contents