pass random value between stateful classes

I'm trying to pass random values between MyFirst class and MySecond class, wich are both stateful.

The code below works fine and you will get two different outputs in the same class depending on wich button you press, but as soon as I try to change one of those final variables to a random variable I get stuck.

Code for random numbers

final random = Random();
  int a, b, sum;

   void changeData() {
    setState(() {
     a = random.nextInt(10);
      b = random.nextInt(10);
      sum = a + b;
    });
  }

My present code looks like this

import 'dart:math';
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'flight 4',
      theme: ThemeData(primarySwatch: Colors.red),
      home: MyFirst(),
      routes: <String, WidgetBuilder>{
        '/screen1': (BuildContext context) => MyFirst(),
        '/screen2': (BuildContext context) => MySecond(),
      },
    );
  }
}

class MyFirst extends StatelessWidget {
  String PassValue = "3 + 2 = ";    //Trying to get this integers to random
  String PassValue2 = "4 - 1 = ";   //Trying to get this integers to random

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('FLIGHT 4'),
       ),
      body: Container(
        height: double.infinity,
        width: double.infinity,
        decoration: BoxDecoration(
          color: Colors.deepOrangeAccent,
        ),
        child: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              RaisedButton(
                  child: Text("+"),
                  onPressed: () {
                    Navigator.push(
                        context,
                        MaterialPageRoute(
                          builder: (context) =>
                              MySecond(passedValue: PassValue),
                        ));
                  }),
              RaisedButton(
                  child: Text("-"),
                  onPressed: () {
                    Navigator.push(
                        context,
                        MaterialPageRoute(
                          builder: (context) =>
                              MySecond(passedValue: PassValue2),
                        ));
                  }),
            ],
          ),
        ),
      ),
    );
  }
}

class MySecond extends StatelessWidget {
  var passedValue;

  MySecond({this.passedValue});

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Text(passedValue),
    );
  }
}