嘿，我刚开始Java，我想做一个简单的计算器，但是我的计算机不工作，你们能帮我吗？

这是我的程序

    import java.util.Scanner;


public class Rechner {

    public static void main(String[] args) {


        int e = 0;
        int z = 0;
        char o = 0;


        Scanner scan = new Scanner(System.in);
        Scanner scan1 = new Scanner(System.in);
        Scanner scan2 = new Scanner(System.in);

        System.out.println("Wie lautet die Rechenaufgabe? : ");



         e = scan.nextInt();
        z = scan2.nextInt();
         o = scan1.next().charAt(0);


       switch(o) {

       case '+' :

          System.out.println("Das Ergebnis lautet: " + (e + z));

         break;

       case '*' :

           System.out.println("Das Ergebnis lautet: " + (e * z));

         break;

       case '-' :

           System.out.println("Das Ergebnis lautet: " + (e - z));

         break;

       case '/' : 

           System.out.println("Das Ergebnis lautet: " + (e / z));

         break;

       default:

           System.out.println("Ungültige Eingabe");



       }






       scan.close();
       scan1.close();
       scan2.close();


       }





    }

当我输入例如5 * 5时，它会说：

Wie lautet die Rechenaufgabe? : 
5*5
Exception in thread "main" java.util.InputMismatchException
    at java.util.Scanner.throwFor(Unknown Source)
    at java.util.Scanner.next(Unknown Source)
    at java.util.Scanner.nextInt(Unknown Source)
    at java.util.Scanner.nextInt(Unknown Source)
    at Rechner.main(Rechner.java:27)

请帮助我了解我该怎么做，并且是初学者

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