def comprova_url(url):
    if url.count(':')!=2:
        return 'error dos punts'
    elif not url[0].isalpha():
        return 'error esquema'
    else:
        pos_dosp = url.find(':')
        part_dreta = url[pos_dosp+1:]
        pos_ultims_dosp = part_dreta.find(':')
        port = part_dreta[pos_ultims_dosp+1:]
        if not port.isdigit():
            return "error port"
        else:
            return "correcta"