import Numeric

d (x,y) (z,w) = sqrt $ (x - z) ** 2 + (y - w) ** 2
 
r n l = if n < 0.001 then return l else do
        s <- (map read . words) <$> getLine :: IO [Double]
        let (x,y) = (s !! 0, s !! 1)
        r (n - 1) ((x,y) : l)

f [] = 0
f (x:[]) = 0
f (x:xs) = foldl (\s p -> s + d x p) 0 xs + f xs

main = do
    n <- read <$> getLine :: IO Double
    l <- r n []
    let x = f l
    print $ x / (n * (n - 1) / 2) * (n - 1)