问当用户输入"2“作为"n”时，我如何获得前两个序列的总和？

EN

public static void main(String[] args) throws UnknownHostException {
        //declarations
        Scanner scan = new Scanner(System.in);
        //to generate this sequence we take the first two as constants and start generating from the third onwards
        double first = 1;//first number in the sequence
        double second = 0.5;//second number in the sequence
        double sum = first+second;//adding first and second
        int n;

        //input
        System.out.println("Enter n: ");
        n = scan.nextInt();
        if(n==1){
           System.out.println("The sum is: " + first);
           return;
        }
        if(n==2){
           System.out.println("The sum is: " + sum);
           return;
        }
        //process
        for(int counter = 2; counter <n; counter += 1)
        {
            sum += 1.0 / Math.pow(counter, 2);//will be computed when you enter values when n is more than 3

        }

        System.out.println("The sum is: " + sum);
     }

public static void main(String[]args) {
     Scanner scan = new Scanner(System.in);
        double sum = 1.5;
        int n;

        //input
        System.out.println("Enter n: ");
        n = scan.nextInt();
        if(n==1)
          System.out.println("The sum is: " + 1);
        //process
        for(int counter = 2; counter <n; counter++) {
            double mul = counter*counter;
            sum += 1.0/mul ;
        }

        System.out.println("The sum is: " + sum);

}

    Enter n: 
    2
    The sum is: 1.5
    Enter n: 
    3
    The sum is: 1.75

import java.util.Scanner;
public class Sum
{
    public static void main(String[] args)
    {
    //declarations

        Scanner scan = new Scanner(System.in);
        double sum = 0;
        int n;

        //input
        System.out.println("Enter n: ");
        n = scan.nextInt();

        //process
        for(int counter = 1; counter <= n; counter += 1)
        {
          if (counter == 1)
            sum = 1;
          else if (counter == 2 )
            sum += 1.0/((counter-1)+(counter-1));
          else 
            sum += 1.0 / ((counter-1)*(counter-1)); 
        }

        System.out.println("The sum is: " + sum);
    }
}