public IActionResult DoUntil(string what, [FromBody] UntilClass until)
 {
     if (what == "sum")
     {
         if (until.Until == null)
         {
             return(Json(new { error = "Please provide a number!" }));
         }
         return(Json(new { result = until.Sum() }));
     }
     if (what == "factor")
     {
         return(Json(new { result = until.Factor() }));
     }
     return(Json(new { error = "Please provide a number!" }));
 }