See here : [https://berryberry.hatenablog.jp/](https://berryberry.hatenablog.jp/entry/2022/03/27/034315)

Opened with Ghidra for the first step.


This program is similar to the Pascal. You can see that the ivar3 variable is compared with the input number in line 37.

In the function1, it calculates the greatest common divisor of local_20 variable and local_28 variable.

In the function2, it calculates the factorial of the ivar3 variable plus 3.
The solution is below.
```
import math
from pwn import *

elf = ELF("./decompile", checksec=False)

host = '143.198.224.219'
port = 21530

def func1(num1, num2):
return math.gcd(num1, num2)

def func2(num):
if (num == 0):
return 1
else:
return func2(num-1) * num

r = remote(host, port)

while (True):
# receive two numbers
now = r.recvline()
if (b"fun() took" in now):
break
a, b = map(str, now.decode('utf-8').split())

# send number which does func1 and func2
num = func2(func1(int(a), int(b))+3)
r.sendline(str(num).encode())

print(now.decode('utf-8'))
r.interactive()
```
You can get the FLAG.