c#, ignore the error in catch (as in try catch)

In a list box, when I click on an item, this code puts the selected item on the screen. One of the items reads <All> which put the program in panic and produces the error below:

Question: How can I find the error number for this instance and pacify the error?

try
{
   Response.Write(lstCity.SelectedItem.Text); 
}
  catch (Exception ex)
{
  // *********find the error number when lstCity.SelectedItem.Text == <All> and pacify the error
}

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Error message when <All> is selected:

Server Error in '/' Application.

A potentially dangerous Request.Form value was detected from the client (lstCity="<All>").

Description: ASP.NET has detected data in the request that is potentially dangerous because it might include HTML markup or script. The data might represent an attempt to compromise the security of your application, such as a cross-site scripting attack. If this type of input is appropriate in your application, you can include code in a web page to explicitly allow it. For more information, see http://go.microsoft.com/fwlink/?LinkID=212874. 

Exception Details: System.Web.HttpRequestValidationException: A potentially dangerous Request.Form value was detected from the client (lstCity="<All>").

Source Error: 


[No relevant source lines]

Source File: c:\Users\Mike\AppData\Local\Temp\Temporary ASP.NET Files\vs\0db659aa\1e56896f\App_Web_1ohlj355.0.cs    Line: 0 

Stack Trace: 


[HttpRequestValidationException (0x80004005): A potentially dangerous Request.Form value was detected from the client (lstCity="<All>").]
   System.Web.HttpRequest.ValidateString(String value, String collectionKey, RequestValidationSource requestCollection) +9693725
   System.Web.<>c__DisplayClass5.<ValidateHttpValueCollection>b__3(String key, String value) +18
   System.Web.HttpValueCollection.EnsureKeyValidated(String key) +9693093
   System.Web.HttpValueCollection.GetValues(String name) +17
   System.Web.UI.WebControls.ListBox.LoadPostData(String postDataKey, NameValueCollection postCollection) +51
   System.Web.UI.WebControls.ListBox.System.Web.UI.IPostBackDataHandler.LoadPostData(String postDataKey, NameValueCollection postCollection) +15
   System.Web.UI.Page.ProcessPostData(NameValueCollection postData, Boolean fBeforeLoad) +303
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +6704
   System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +245
   System.Web.UI.Page.ProcessRequest() +72
   System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +21
   System.Web.UI.Page.ProcessRequest(HttpContext context) +58
   ASP.default_aspx.ProcessRequest(HttpContext context) in c:\Users\Mike\AppData\Local\Temp\Temporary ASP.NET Files\vs\0db659aa\1e56896f\App_Web_1ohlj355.0.cs:0
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +341
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +69

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.34237

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