#include "stdio.h"
#include "conio.h"
#include "iostream"
#include "math.h"
using namespace std;


void main()
{
    int GP,P,GT,S,y=1;
    scanf("%i",&GP);scanf("%i",&P);scanf("%i",&GT);scanf("%i",&S);
    if((GP+P>=GT)||(GP>=GT))
    {
        if(GP>=GT) printf("%i",GP);
        else
            if(GP+P>=GT) printf("%i",GT);
    }
    else
    {
        while(y!=0)
        {
            if((GP<GT)&&(y!=0)&&(GP+P<=GT)) GP+=P;
            else y=0;
            if((GT>GP)&&(y!=0)&&(GT-S>=GP)) GT-=S;
            else y=0;
        }
        //if(GP>=GT)printf("%i",GP);
        //else printf("%i",GT);
        if(GP+P<=GT) printf("%i",GP);
        else
        {
            if(GT-S<=GP) printf("%i",GP);
            else printf("%i",GT);


        }
    }
    getch();
}