# include <bits/stdc++.h>
# include <atcoder/all>

int main() {
   long A, a, B, b;
   std::cin >> A >> B >> a >> b;
   auto [y, z] = atcoder::crt({a, b}, {A, B});
   long ans = ((y % z) + z) % z;
   std::cout << ans << std::endl;
}