System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.FormatException: Input string was not in a correct format.
   at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
   at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)
   at lambda_method(Closure )
   --- End of inner exception stack trace ---
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
   at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
   at System.Delegate.DynamicInvokeImpl(Object[] args)
   at System.Data.Linq.CommonDataServices.GetKeyFromPredicate(MetaType type, Dictionary`2 keys, Expression mex, Expression vex)
   at System.Data.Linq.CommonDataServices.GetKeysFromPredicate(MetaType type, Dictionary`2 keys, Expression expr)
   at System.Data.Linq.CommonDataServices.GetKeyValues(MetaType type, LambdaExpression predicate)
   at System.Data.Linq.CommonDataServices.GetCachedObject(Expression query)
   at System.Data.Linq.SqlClient.SqlProvider.GetCachedResult(Expression query)
   at System.Data.Linq.SqlClient.SqlProvider.System.Data.Linq.Provider.IProvider.Execute(Expression query)
   at System.Data.Linq.DataQuery`1.System.Linq.IQueryProvider.Execute[S](Expression expression)
   at control_study.Page_Load(Object sender, EventArgs e) in d:\Websites\newexplore\study.aspx.cs:line 20