use while loop to output the sum of all even numbers

i need to prompt the user to input 2 integers: firstNum & secondNum (firstNum must be less than secondNum).
but my main problem is that i got the unexpected output when i try to output the sum of all even numbers between these 2 integers.
Here's my codes:

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

public class EvenNumber
{

      static BufferedReader keyboard =
                        new BufferedReader(new InputStreamReader(System.in));
      
      public static void main(String[] args) throws IOException
      {
            int firstNum, secondNum, n, sum, rem;
            
            n = 0;
            sum = 0;
            
            System.out.print("please enter an integer: ");
            System.out.flush();
            firstNum = Integer.parseInt(keyboard.readLine());
            
            System.out.print("please enter another one that is larger than the first integer: ");
            System.out.flush();
            secondNum = Integer.parseInt(keyboard.readLine());
            
            rem = firstNum%2;
            
            while(firstNum < (secondNum - 2))
            {
                  if(rem == 0)
                  {
                        {
                              firstNum = firstNum + 2;
                              n++;
                              {
                                    sum +=firstNum;
                              }
                        }
                        
                        System.out.print(sum);
                  }
                  
                  
                  else
                  {
                        {
                              firstNum = firstNum + (2*n + 1);
                              n++;
                              {
                                    sum += firstNum;
                              }
                        }
                        
                        System.out.print(sum);
                  }
            }
      }
}


i input 3 and 17, then the output should be 47,
but it outputs "4112372".
please help me to check my codes.
thanks