Different results using same function from different libraries

Here is some code:

#include <iostream>
#include <cmath>

using namespace std;

int main()
{
    long long int a, b;
    long long int c;

    cin >> a >> b;

    c = abs(a) + abs(b);
    cout << c;
    cout << endl;
}

Which supposed to return 1000000000000000002 when I input 1000000000000000000 and 2.

If I try to do it with cmath it will return 1000000000000000000, but if I use cstdlib it will return 1000000000000000002. Why is that even happening?

Also considering that I'm using cmath, shouldn't it work even more proper?

I'm using Linux Mint 18.2 64bit, Eclipse Platform.