Sum of odd numbers

Question

You are given the sum 1 - 3 + 5 - 7 + 9 - 11 + 13... You should compile a program that (given integer N) finds and displays the value of the sum to the N-th addend.

I don't even have an idea how this program should be. I've written some code but don't know what to add. Please, can you help me? :)

Here is my code:

Scanner input = new Scanner(System.in);

System.out.print("n = ");
int n = input.nextInt();
int sum = 0;

for (int i = 1; i <= n; i++) {       
    if (i % 2 != 0) {
        sum = sum + i;
    }
}

System.out.println(sum);

Answer 1

May be you want this

If I enter i/p 7 this will produce -4 as o/p

for (int i = 1; i <= n; i+=2) {
        if( i % 4 == 1 )
            sum = sum + i;
        else
            sum = sum - i;
}

in @fafl style (Using Ternary operator), correct me if I'm wrong

sum += (i % 2 != 0) ? ( i % 4 == 1 ) ? + i : - i;

If I enter i/p 7 this will produce 7 as o/p

        int n = input.nextInt();
        int sum = 0;
        int addOrDedduct = 1;
        for (int i = 1; i <= n; i++ ) {
                if( addOrDedduct % 4 == 1 )
                    sum = sum + addOrDedduct;
                else
                    sum = sum - addOrDedduct;
                addOrDedduct+=2;
        }
        System.out.println(sum);

Update:
fafl's statement sum = n % 2 == 0 ? -n : n producing same o/p, Here you don't need to use loop
Forget about the loop and use fafl's answer.