/* -*- coding: utf-8 -*-
 *
 * 2558.cc:  No.2558 中国剰余定理 - yukicoder
 */

#include<cstdio>
#include<algorithm>
 
using namespace std;

/* constant */

/* typedef */

/* global variables */

/* subroutines */

template <typename T>
void exgcd(T x, T y, T &a, T &b, T &c) {
  T r0 = x, r1 = y;
  T a0 = 1, a1 = 0;
  T b0 = 0, b1 = 1;

  while (r1 > 0) {
    T q1 = r0 / r1;
    T r2 = r0 % r1;
    T a2 = a0 - q1 * a1;
    T b2 = b0 - q1 * b1;
    r0 = r1, r1 = r2;
    a0 = a1, a1 = a2;
    b0 = b1, b1 = b2;
  }
  c = r0;
  a = a0;
  b = b0;
}

/* main */

int main() {
  int a, b, ra, rb;
  scanf("%d%d%d%d", &a, &b, &ra, &rb);

  int u, v, w;
  exgcd(a, b, u, v, w);
  //printf("%d * %d + %d * %d = %d\n", a, u, b, v, w);

  int x = ra * b * v + rb * a * u;
  while (x < 0) x += a * b;

  printf("%d\n", x);

  return 0;
}