How can I make it so that the int method can return an int and string value?

Question

C++ / C#

The task itself:
Develop a class library containing a class with a method that will simulate the work of a robot calculator. The input to the robot is a number and a string - a sequence of commands, where "0" means "add 2 to the number",
"1" means "multiply the number by 5".
Each time the command is applied to the already obtained result.
It is necessary to return something like "the wrong command is entered".

My code :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace RobotLib
{
    public class Robot
    {
        string comand;
        int num;
        

        public void setComand(string a)
        {
            comand = a;
        }
        public string getComand()
        {
            return comand;
        }
        public void setNum(int b)
        {
            num = b;
        }
        public int getNum()
        {
            return num;
        }
        public int robotAction()
        {
          
            if(comand == "0")
            {
               return num + 2;
            }
            else if(comand == "1")
            {
                return num * 5;
            } 
      
        }
    }
}

Answer 1

It is necessary to return something like "the wrong command is entered".

This is solved with Exception. Those. Report an error by throwing an exception.