Monistus

import java.util.*;

class Monistus {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        StringBuilder input = new StringBuilder(sc.nextLine());
        sc.close();

        int i = 0;
        while (i < input.length()){
            if(Character.isDigit(input.charAt(i))){
                int num = Character.getNumericValue(input.charAt(i));
                String mid = input.substring(i+1, i+num+1);

                input.replace(i, i+num+1, mid+mid);
                i -= 1;
            }
            i += 1;
        }
        System.out.println(input);
    }
}

Test 1

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 2

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 3

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 4

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 5

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 6

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 7

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 8

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 9

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 10

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 11

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 12

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 13

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 14

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...