Java Programming

I have to compute the sum of 1/2 - 1/3 + 1/5...+ (-1)^n+1 * 1/p (where p is the nth prime)
I have done a similar program where i compute the sum 1- 1/2 +1/3...(-1)^n+1 * 1/n. that was fairly simple with a for loop, i've tried to incorporate the design of that program into the new one but with no luck. the old program is

import java.util.Scanner;

public class Project4 {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
/**
* Get input from user.
*/
System.out.println("Enter an integer");
int n = input.nextInt();
double newNum;
double s = 0;
/**
* create a loop
*/
for(int i = 1; i <= n; i++){
/**
* make conditions
*/
if (i%2 == 0)
newNum = -1 * (1.0/i);
else
newNum = 1.0/i;
/**
* add the sum
*/
s = s + newNum;
}
/**
* print sum
*/
System.out.println(" The sum is " +s);

}

}


the new one i've been trying to make hasn't worked like i planned so i've been messing with variables and whatnot so now it looks kinda messed up and well...its probably useless. i know what a prime number is, where it is only divisible by 1 and itself and i have been reading other material on here about primes but none of it is quite like my situation, any help would be great (my first programming class)