fraction class in java

I created a fraction Java class but it gives wrong results. Sometimes it gives good answers and sometimes bad or maybe it doesn’t give any results just random digits? Any help or hints are welcome

import java.util.Scanner;

public class Ulamek{
    public static void main(String[] args){

        Scanner input = new Scanner(System.in);
        System.out.println("Podaj licznik i mianownik pierwszego ulamka: ");
        Fraction ulamek1 = new Fraction(input.nextInt(), input.nextInt());
        System.out.println("Podaj licznik i mianownik drugiego ulamka: ");
        Fraction ulamek2 = new Fraction(input.nextInt(), input.nextInt());
        System.out.println("Jaka operacje chcesz wykonac? ( +, -)");
        char c = input.next().charAt(0);
        System.out.print(ulamek1);
        System.out.print(" " + c + " ");
        System.out.println(ulamek2);
        switch(c){
            case '-':
            System.out.println(ulamek1.sub(ulamek2));
            break;
            case '+':
            System.out.println(ulamek1.sum(ulamek2));
            break;
        }
    }
}

class Fraction{
    private int numerator;
    private int denominator;

    public Fraction(int numerator, int denominator){
        if(denominator < 0){
           this.numerator = -numerator;
           this.denominator = -denominator;
        }else{
              this.numerator = numerator;
              this.denominator = denominator;
        }
    }

    public Fraction sum(Fraction other){
        int nww =  nww(other.denominator, this.denominator);
        int newNumerator = nww/this.denominator*this.numerator + nww/other.denominator*numerator;
        return new Fraction(newNumerator, nww).reduce();
    }

    public Fraction sub(Fraction other){
        int nww = nww(this.denominator, other.denominator);
        int newNumerator = nww/this.denominator*this.numerator - nww/other.denominator*other.numerator;
        return new Fraction(newNumerator, nww).reduce();
    }

    public Fraction reduce(){
        int nwd = nwd(this.numerator, this.denominator);
        return new Fraction(this.numerator/nwd, this.denominator/nwd);
    }

    private int nwd(int a, int b){
        if(a < 0){
            a = -a;
        }
        if(b < 0){
            b = -b;
        }
       while(b != 0){
          int temp = b;
          b = a % b;
          a = temp;
       }
       return a;
    }

    private int nww(int a, int b){
        return a*b/nwd(a, b);
    }

    public String toString(){
        return numerator + "/" + denominator;
    }

}

I can’t find a mistake, could anyone help me find it? I have just started learning Java so I am very beginner.

>Solution :

A typo. You left out the other. on this line

int newNumerator = nww/this.denominator*this.numerator + nww/other.denominator*numerator;

It should read

int newNumerator = nww/this.denominator*this.numerator + nww/other.denominator*other.numerator;