The answer is correct, but the task is failed.

Great thanks, Guadalupe!

I failed a bunch of tasks myself because I liked to use static variables too.

Yes. But I am still curious: is the use of static variables when you can avoid it a bad programming habit/style?

The simple answer is yes, but it is a complex issue and they are very useful when needed. Here is an example of your code modified for this example:

public class Solution {
    public static void main(String[] args) {
        Solution mySolution1 = new Solution();
        Solution mySolution2 = new Solution();

        mySolution1.myMethod(10);

        System.out.println(mySolution2.sum);

        mySolution2.myMethod(10);
    }

    static int sum = 1;

    static void myMethod(int n) {
        for(int i = 1; i<n; i++) {
            sum = sum*(i+1);
        }
        System.out.println(sum);
    }
}

I created 2 objects from your blueprint. Each object should have sum and myMethod. I called myMethod from mySolution1 with argument of 10 and received the proper response of 362880. Everything is good. Then I tried doing the same with mySolution2, but the output was -180289536... which is not good. The reason why is static variables are class wide variables, meaning they are shared between every object created. (i also output the value of mySolution2.sum just prior to calling the method to show this). Copy and paste this code in and see the results. I don't know if this makes a whole lot of sense at this point, but to show you the difference between static variables and non-static modify the above code my removing the static modifier from the int and the method:

int sum = 1;
void myMethod(int n) {

and run it again. The result will show that it runs properly and both method calls output: 3628800

I bet the codegym verification system takes your code and creates a couple objects from it like i did and tests the result of each.

how to put it inside&

How to move line 13 inside the method?