This chall is about File Descripter(A.K.A fd)
Using IDA, you can see below code.
```
int __cdecl main(int argc, const char **argv, const char **envp)
{
char buf; // [rsp+0h] [rbp-40h]
int v5; // [rsp+38h] [rbp-8h]
int v6; // [rsp+3Ch] [rbp-4h]

puts("enter a number");
fflush(_bss_start);
__isoc99_scanf("%d", &v5;;
v6 = open("flag", 0) + 1001;
read(v5 - 1085645, &buf, 0x17uLL);
puts(&buf;;
return 0;
}
```
If the read function works correctly, we can get flag file.
So, What we have to do is just calculate the v5 variable.