code taxi

#include <iostream>
#include <cstdlib>
#include <string>
#include <cstdio>
#include <locale>
#include <algorithm>
#include <cmath>
#include <vector>
#include <sstream>
#include <string>
#include <map>
#include <set>

#define pi 3.14159265358979323846
#define ll long long

//#include <bits\stdc++.h>
using namespace std;

int main(){
int a,b,c,d;
cin >> a >> b >> c >> d;
if(c <= a) {
cout << a <<endl;
return 0;
}
else{
	while(c-d >= a){
		c-=d;
		a+=b;
	}
	cout << a <<endl;
}
}