How to calculate working hours?

How to calculate working hours?

I'm struck at formula field.
i have given the coding to calculate the total working hours, by giving in time and out time in date/time format.
the logic is working correctly. but the format to display is somewhat wrong. The coding i have given is


Concat(Tostring(Floor((Datecomp(${Performance.New Out Time},${Performance.New In Time}))/60)),':',Tostring(((Datecomp(${Performance.New Out Time},${Performance.New In Time}))/60-Floor((Datecomp(${Performance.New Out Time},${Performance.New In Time}))/60))*60))

Inputs: New In Time 09/10/2017 08:46
New Out Time 09/10/2017 18:04

Output: New Working Hours 9.0:18.0

But i want the output as 9:18