RDLC - how to sum column of type string in footer?

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ASP.NET

RDLC

A.O.A
i have a report that shows Overtime of a employee of particular month.
i want to calculate the total no. 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 in runtime