var
   u1, u2, d1, d2: longint;
K: real;
begin
   readln(u1, d1, u2, d2);
   if u1>u2 then write(u1)
      else
begin
k:= (u2-u1)/(d1+d2);
write(int(k)*d1 +u1 + d1:0:0);end;
end.

Check these cases

1 4 2 2
answer: 2

1 3 5 2
answer: 4

I think the answer is 5
because Petr's offer 1 + 4 = 5
driver's offer = 2 - 2 = 0