I was send solution for this problem in Java and receive WA - why?

My solution is equal to example in FAQ:

import java.io.*;
import java.util.*;

public class Sum
{
   public static void main(String[] args)
   {
      Scanner in = new Scanner(System.in);
      PrintWriter out = new PrintWriter(System.out);

      int a = in.nextInt();
      int b = in.nextInt();
      out.println(a + b);

      out.flush();
   }
}