text: uses tools_p;
  
  Const
      zero=1e-4;
      phi=(sqrt(5)-1)/2;
  
  Var
      L,h,t:extended;
  
  procedure work;
  var lastf,lastx,x,f:extended;
  begin
      lastx:=t*phi;
      lastf:=ask(lastx);
      while abs(h-t)>zero do
        begin
            x:=h+t-lastx;
            f:=ask(x);
            if f>lastf then begin
                if x>lastx then h:=lastx else t:=lastx;
                lastx:=x; lastf:=f;
            end
            else begin
                if x>lastx then t:=x else h:=x;
            end;
        end;
  end;
  
  Begin
      L:=Start;
      h:=0; t:=L;
      work;
      Answer((h+t)/2);
  End.