I dont know what's wrong
public class Solution {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

        int input = Integer.parseInt(reader.readLine());
        reader.close();

        System.out.println(factorial(input));
    }

    public static String factorial(int n) {
        //write your code here
        String s = "";
        if (n <0)
            s = "0! = 1";
        else if(n == 0)
            s = "1";
        else {
            int factorial = 1;
            for (int i = 1; i <= n; i++) {
                factorial = factorial * i;
            }
             s = Integer.toString(factorial);
        }
        return s;

    }
}