Scanf.scanf "%f %f %f" (fun c rin rout ->
    let r1 = (rout -. rin) /. 2. in
    let r2 = (rout +. rin) /. 2. in
    let pi = 3.1415926535897932384626 in

    Printf.printf "%.8f\n" @@ c *. (r1 *. r1 *. pi) *. 2. *. pi *. r2
)