Concatenate based on criteria

In column A i have numbers which indicates how many columns should be concantenate.

For ex:
If A = 3 (it will concantenate B, C and D) or
If A = 5 (it will concantenate B, C, D, E and F)


I need it conditionally because I have 50 columns to concatenate and I do not want to be like this (ex: 5100331559,5100331554,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,) when there is only two columns to concantenate