How to sum column of type string in footer?

A.O.A,

I have a report that shows Overtime of an employee of a particular month.
I want to calculate the total number of overtime in footer

This is how i'm calculating overtime

• "tout" is EmployeeLeavingtime


• "otLimint" is the Timelimit (like if ShiftEndTime is 6:00PM then overtime will be
  calculated if tout is > 6:30)



• shiftendTime is (6:00PM)

  
  
  

  string timeOut = r["TimeOut"].ToString();
   Time tOUT = new Time(timeOut);
   Time OtLimit = new Time(18, 30, 00);
   
   if (tOUT > OtLimit)
   {
       DateTime ShiftendTime = Convert.ToDateTime(r["ShiftEndTime"].ToString());
       DateTime Out          = Convert.ToDateTime(r["TimeOut"].ToString());
       TimeSpan span         = Out.Subtract(ShiftendTime);
       r["OTHrs"] = span.ToString().Replace("-","");
   }
   

im using

Sum(Fields!TotalLH.Value) in Report footer expression
 

but this is not showing Total at runtime

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