为什么第七行加个int 声明，代码就报错

public class HelloWorld{
    public static void main(String[] args) {
        int one = 10 ;
        int two = 20 ;
        int three = one + two ;
        System.out.println(""+three);
        int three += one;
        System.out.println(""+three);
         three -= one;
        System.out.println(""+three);
         three *= one;
        System.out.println(""+three);
        three /= one;
        System.out.println(""+three);
        three %= one;
        System.out.println(""+three);