> For the complete documentation index, see [llms.txt](https://centinels.gitbook.io/home/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://centinels.gitbook.io/home/writeups/n00bzctf/misc-subtraction.md).

# misc/subtraction

### Writeup by Aryan

This is the source code we are provided with.

```python
import random

n = 696969

a = []
for i in range(n):
    a.append(random.randint(0, n))
    a[i] -= a[i] % 2

print(' '.join(list(map(str, a))))

for turns in range(20):
    c = int(input())
    for i in range(n):
        a[i] = abs(c - a[i])

    if len(set(a)) == 1:
        print(open('/flag.txt', 'r').read())
        break
```

If we keep subtracting by the mean of the dataset, the numbers will eventually all equal each other. The proof is left as an exercise to the reader.

```python
import socket
import statistics

def main():
    host = 'challs.n00bzunit3d.xyz'
    port = 10029

    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
        s.connect((host, port))
        data = b''
        while b'\n' not in data:
            data += s.recv(4096)
        data = data.decode().strip()
        numbers = list(map(int, data.split()))
        print(f"Received numbers: {len(numbers)}")

        for turn in range(20):
            print(f"Iteration {turn + 1}:")
            c = int(statistics.mean(numbers))
            s.sendall(f"{c}\n".encode())
            print(f"Sent value of c: {c}")
            numbers = list(set(abs(c - x) for x in numbers))
            print(f"Amount of numbers: {len(numbers)}")
            
            if len(set(numbers)) == 1:
                response = b''
                while b'\n' not in response:
                    response += s.recv(4096)
                response = response.decode().strip()
                print(response)

if __name__ == "__main__":
    main()

```

Flag: `n00bz{1_sh0uld_t34ch_my_br0th3r_logs_e18be9bd2874}`