I’ve been trying the cs50 credit problem and i am stuck in the part where you add every other digit in the number. This is what i came up with.

#include <iostream>
using namespace std;

int main() {
    int num = 123456789;
    int sum, last, second;
    while (num>0)
    {
        last = num/10;
        second = last%10;
        num /=10;
        cout<<second<<endl;
        
        sum += second;
        num /=10;
    }
    cout<<"sum: "<<sum<<endl;

    return 0;
}

Output:
8
6
4
2
0
sum: 20

It works and gives the sum of every other digit which is 20 in this case. But it gives a very different answer when i add more values and change it to long.

This is where it gives a different result.

#include <iostream>
using namespace std;

int main() {
    long num = 123456789;
    long sum, last, second;
    while (num>0)
    {
        last = num/10;
        second = last%10;
        num /=10;
        cout<<second<<endl;
        
        sum += second;
        num /=10;
    }
    cout<<"sum: "<<sum<<endl;

    return 0;
}

Output:
8
6
4
2
0
sum: 140736744828580

The answer should be 20 but it gives a really big number. I think the problem is my computation in the while loop but I can’t seem to find a solution which is why I posted it here. Thanks in advance.