increment operator: what's the output?

Question

What's the output of this code?
The right answer may be surprising to you!

I'll post the right answer if no one answers. in a a week.

#include <iostream>
using namespace std;

int main()
{
int i = 5;
i = ++i + i++;
cout<<i<<endl;

return 0;
}

Answer 1

13

Answer 2

undefined. It depends on the compiler. 12 or 13.