#include<bits/stdc++.h>
#include <boost/integer/extended_euclidean.hpp>
using namespace std;
using namespace boost::integer;

int main() {
    long long A, B, a, b;
    cin >> A >> B >> a >> b;
	
	auto res = extended_euclidean(A, B);
    long long ans = ((A * res.x) * (b - a) + a) % (A * B);
    ans = (ans + (A * B)) % (A * B);
    
    cout << ans << endl;
}