fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)


val () =
    let
        val c = Real.fromInt (readInt ())
        val r_in = Real.fromInt (readInt ())
        val r_out = Real.fromInt (readInt ())

        val r1 = (r_out - r_in) / 2.0
        val r2 = (r_out + r_in) / 2.0
                        
        val ans = 2.0 * Math.pi * Math.pi * r1 * r1 * r2 * c
    in    
        print ((Real.fmt (StringCvt.FIX (SOME 10)) ans) ^ "\n")
    end