How Can I summarize a number field in crystal report ?

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CrystalReports

I'im trying to calculate a sum of workedhours:
@TotalWorkedHours:

numbervar fin_t := {@SumTotalSec2WithoutGroup};
NumberVar Hours   := Truncate (fin_t/3600);
NumberVar Minutes := Truncate (Remainder (fin_t,3600)/60);
NumberVar Seconds := Remainder (fin_t,60) ;
Totext (Hours,0,"") + ':'+ Totext (Minutes,0,"") + ':' + Totext(Seconds,0,"")

@SumTotalSec2WithoutGroup: I get a problem here "This field cannot be summarized"

SUM({@TotalSecAchivedHour})

@TotalSecAchivedHour:

local stringvar array completetime;
local numbervar totalseconds;
completetime:=split({@achievedworkedhours},":");
totalseconds:= (3600*cdbl(completetime[1]))
 + (60*cdbl(completetime[2]))
;

@achievedworkedhours: Time value with string type (hh:mm)

if IsNull({@PermissionHoursInTime})
then {@SumWorkedHours}
else {@TotalHoursWithPermission}

I get A proble in the formula "@SumTotalSec2WithoutGroup" :
error message : "This field cannot be summarized"
Any help please ?