Some web challenges require executing /readflag to get the flag. This executable will ask the user to compute some simple math. The timeout is very short, so the intended way to solve it is to write a script. However, there are better ways to solve it:

Solution 1: Trap the SIGALRM signal

$ trap "" 14 && /readflag 
Solve the easy challenge first (((((-623343)+(913340))+(-511878))+(791102))-(956792)) 
input your answer: -387571 
ok! here is your flag!! 

Solution 2: mkfifo trick

$ mkfifo pipe
$ cat pipe | ./readflag |(read l;read l;echo "$(($l))" > pipe;cat)
input your answer: 
ok! here is your flag!! 

Ref: https://balsn.tw/ctf_writeup/20190427-*ctf/#solve_readflag-(not-a-challenge)

sudo dpkg --add-architecture i386
sudo apt update
sudo apt install libc6:i386 libncurses5:i386 libstdc++6:i386
sudo apt install gcc-multilib g++-multilib # for compiling