public class Fraction
{
    // fields
    int num = 0;
    int denom = 1;
    
    // constructor (same name as class, no return type)
    Fraction(int num2, int denom2)
    {
        if ( (num2 * denom2) > 0)
        {
            num = Math.abs(num2);
            denom = Math.abs(denom2);
        }
        else if ( num2 * denom2 < 0)
        {
            num = -Math.abs(num2);
            denom = Math.abs(denom2);
        }
    }
    
    Fraction add(Fraction f)
    {
        int num2 = num*f.denom + f.num*denom;
        int denom2 = f.denom*denom;
        return new Fraction(num2, denom2);
    }
    
    void display()
    {
        System.out.println(num + "/" + denom);
    }
    
    public String toString()
    {
        if (denom == 0)
            return "invalid";
        else
            return num + "/" + denom;
    }
}