#include <bits/stdc++.h>
using namespace std;
 
int main() {
 long long a,b,n,m;
 cin >> a >> b >> n >> m;
 if(a>b){
     int p = (a-b)/n;
     while(a>b){
         a-=p;
         b+=p;
     }
     cout << a << endl;
 }
 else{
     int q = (b-a)/m;
     while(b>=a){
         a+=q;
         b-=q;
     }
 }
 }