NUMBERX code c++

#include <bits/stdc++.h>
using namespace std;
long long lcm(long long a, long long b)
{
    return a*b/__gcd(a,b);
}
int main()
{
    freopen("numberX.inp","r",stdin);
    freopen("numberX.out","w",stdout);
    long long a,b;
    cin >> a >> b;
    if (a<b) swap(a,b);
    if (a%b==0)
    {
        if (a==b) cout << a;
        else cout << a-b;
    }
    else cout << lcm(a,b)-a-b;
}