照着 README 跑，第几步会卡住

README 给了四步，而仓库里实际只有 main.py、calc.py、README.md。运行下面这段程序，看第几步会失败：

def dry_run(steps, files):
    for i, (cmd, need) in enumerate(steps, 1):
        if need and need not in files:
            return i, need
    return 0, None

steps = [('git clone https://example.com/calc.git', None),
         ('cd calc', None),
         ('pip install -r requirements.txt', 'requirements.txt'),
         ('python main.py', 'main.py')]
files = ['main.py', 'calc.py', 'README.md']
i, need = dry_run(steps, files)
print(str(i) + "/" + str(need))