ช่วยทำการบ้านหน่อยครับ หาแล้วได้ 0 ตลอดเลยครับ......................

import java.util.*;
class HelloWorld {

 public static void main(String[] args)
 {
  Scanner data = new Scanner(System.in);
  System.out.print("Input odd number only : ");
  int n = data.nextInt();
  int j;
  int temp = 0;
  double avg;
  double op = 0;
  if(n%2==0)
    System.out.println("Error");
  else
    {for(j=1;j<=n;j+=2)
       {long powans = power(j);
        long facans = factorial(j);
        avg = powans/facans;
        temp = temp+1;
        if(temp%2==0)
           {op = op-avg;
           /*System.out.print(op+"-("+powans+"/"+facans+")");*/}
        else 
           {op = op+avg;
           /*System.out.print(op+"+("+powans+"/"+facans+")");*/}
		   System.out.println(op+" "+powans+" "+facans+" "+avg); 
       }
       System.out.println("="+op); 
      }}
 public static long power(int j)
 {long powans = 1;
  for(int i=1;i<=2;i++)
     {powans = powans*j;}
      return powans;
 }
 public static long factorial(int j)
 {long facans = 1;
  for(int i=1;i<=j;i++)
     {facans = facans*i;}
      return facans;
 }

}