string text = instance.ActiveTab.FindElementByXPath("//label[@for='gb_bottest']",0).GetAttribute("innertext");
for(int i = 0; i < text.Length; i++)
{
if(Char.IsDigit(text[i]))
{
text = text.Substring(i, text.Length-i);
var res = new DataTable().Compute(text, null);
instance.ActiveTab.FindElementByXPath("//input[@id='gb_bottest']",0).SetValue(res.ToString(),"full",false);
}
}