The main page looked like http://petushok.2018.ctf.kaspersky.com/?logo=petushok.png
The picture name can be changed to http://petushok.2018.ctf.kaspersky.com/?logo=main.py. The main.py name was guessed. There is an import cococo, lets get it.
http://petushok.2018.ctf.kaspersky.com/?logo=cococo.py.

We have check function there, the task is to find the check input which gives 1.

The solver:

```
def check(value):
# a lot of code
return (a140574420399240, a140574420399456, ..., (a140574419586512 + 1), (a140574419608064 + 1))

def brute_from(n, condition_idx):
end_pos = (condition_idx + 104) % 137

if condition_idx >= 300:
print("flag", hex(n))
exit()

q = check(n)
if q[condition_idx % 137] == 1:
brute_from(n, condition_idx+1)

n ^= 1 << end_pos
q = check(n)
if q[condition_idx % 137] == 1:
brute_from(n, condition_idx+1)

brute_from(0, 0)

```