S = 2 – 4 + 6 – 8 + …………To n

import java.util.*;
class sum_series
{
    public static void main(String args[])
    {
        int a=1,s=0,n;
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the value of n");
        n=sc.nextInt();
        for(int i=2;i<n;i=i+2)
        {
         if(a%2==0)
         s=s-i;
         else
         s=s+i;
         a++;
         
        }
        System.out.println("SUM="+s);
      
       
    }
}