How to find the value of this mathematical expression?

Question

Mathematics

For example, if N = 6
How to describe the arithmetic in detail?
see picture

Answer 1

something like this:

int foo(int n)
{
int sign =1;
int sum = 0;
for(int i = 0; i <= n; i++)
{
  sum += sign * (2 * i + 1);
  sign *= -1;
}
return sum;
}

Answer 2

int f(int n){
   return 1 + n;
}

Do not thank.