Problem in a math related question of codeforces div2 D

help
1

can anyone plz explain this solution (based on inclusion exclusion principle )

or
Submission #69763365 - Codeforces (not on inclusion exclusion but still kind of same)

2

This is Euler’s Totient Function. link

3

that’s 9th grade math tho
#FIITJEE

4

i qualified RMO XDDD