First we look at the html, 
there we can see a comment:
 there you can see Selina our neighborhood superhero used eval,
so we try a known injection, we inject: system("ls")
and as is suppous to because once again the superheroes lost we can see Selina's files
flag.txt index.php index.php
we can see that there is a flag.txt so we cat it.
system("cat flag.txt")
from here we can just copy and paste the flag: KCTF{WaS_mY_cAlCuLaToR_sAfE}
all that is left is to submit a report to Selina, the report:
"No Selina, your calculator wasn't safe :("
Flag: KCTF{WaS_mY_cAlCuLaToR_sAfE}