#include<iostream>
#include<math.h>
#include<algorithm>
#include<string.h>

using namespace std;
int a,b,c,d;
int main()
{
    cin >> a >> b >> c >> d;
    int h = c;
    if(a >= c)
    {
        cout << a;
        return 0;
    }
    while(1)
    {
        if(a == c)
        {
            cout << a;
            return 0;
        }
        a+=b;
        if(c-d >= a) c-=d; else
        {
            if(a > h) cout << h; else
            cout << a;
            return 0;
        }
    }
    return 0;
}