#!/usr/bin/env python3
import socket

START, END = 8000, 8100

def port_free(port):
    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
        try:
            s.bind(("0.0.0.0", port))
            return True
        except OSError:
            return False

for port in range(START, END + 1):
    if port_free(port):
        print(port)
        exit(0)

print("ERR_NO_FREE_PORT")
exit(1)