var S, K, A, B, N, rez:longint;
begin
    readln (S, K, A, B);
    if s*a=b then writeln (s*k)
             else
                 begin
                      n:=1;
                      while n<1001 do
                      begin
                           if (abs(s*a-b)) mod n =0 then
                              begin
                                   rez:=((b-s*a) div n) +s*k;
                                   if (rez>=0) and (a>n*k) then
                                                              begin
                                                                   writeln (rez);
                                                                   halt;
                                                              end;
                              end;
                           inc (n);
                      end;
                      if n=1001 then writeln (-1);
                 end;
end.

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