import Control.Monad (replicateM)

-- Enter your code here. Read input from STDIN. Print output to STDOUT

solve pairs = sum lengths
            where lines                    = zip pairs (tail pairs ++ [head pairs])
                  dst ( (x1,y1), (x2,y2) ) = ((x1 - x2)**2 + (y1 - y2)**2)**0.5
                  lengths                  = map (dst) lines
main = do
    let toInt s  = read s :: Int
    let toDoub s = read s :: Double
    lines <- getLine >>= (\line -> return $ toInt line) >>= (\n -> replicateM n getLine)
    let pairs = map (\[x,y] -> (toDoub x, toDoub y)) . map words $ lines
    print (solve pairs)