open System
let R T = stdin.ReadLine()|> T
let RS T = stdin.ReadLine().Split()|>Array.map T
let Mod = (1e9|>int64)+7L
[<EntryPoint>]
let main argv =
    let dist x1 y1 x2 y2 = 
        let dx = (x1-x2)|>double
        let dy = (y1-y2)|>double
        sqrt (dx*dx+dy*dy)
    let N = R int
    let x = Array.zeroCreate N
    let y = Array.zeroCreate N
    for i in [0..N-1] do
        let p = RS double
        x.[i] <- p.[0]
        y.[i] <- p.[1]
    [0..N-2]
    |>Seq.sumBy(fun c -> [c..N-1]|>Seq.sumBy(fun d -> dist x.[c] y.[c] x.[d] y.[d]))
    |>fun c ->  c*(2|>double)/(N|>double)
    |>stdout.WriteLine
    0
