program bs;
var
  ans,count,max,counto,countx,maxo,maxx:int64;
begin
  assign (input,'bs.in');
  reset (input);
  assign (output,'bs.out');
  rewrite (output);
  while not(eof(input)) do begin
    readln (counto,countx,maxo,maxx);ans:=0;
	if counto<countx then count:=counto else if counto=countx then count:=counto-1 else count:=countx;
	if maxo<maxx then ans:=count*maxo+(count+1)*maxx else ans:=count*maxx+(count+1)*maxo;
	writeln (ans);
  end;
  close (input);
  close (output);
end.