var a,b,c,d,cen,k:integer;
begin
read(a,b,c,d);
while k<>1 do begin
if (a+b>c) then begin
k:=1;cen:=c;end
else a:=a+b;
if(c-d<a)then begin
k:=1;cen:=a;end
else c:=c-d;end;
writeln(cen);
end.