addition of two numbers after accepting from user

Question

if i do int sum=a+b; in case of where i m adding two numbers after taking from user then it is showing incorrect answer
can you please tell the reason behind it

Answer 1

int a, b;
std::cin >> a>> b;
int sum= a+b;
std::cout <<sum;

Answer 2

It's generally wrong to sum before receiving values.
You're most likely to get 0.

Answer 3

I know that it will work in this manner but I am not able to understand why it's not working if we definitely sum before accepting valued