data = []
data.append("user 54321")
print('Welcome!')
def main1():
    n = 3
    b = 5
    while (n != 0):
        str = []
        a = input('Select:\n1.Registration.\n2.Entry.\n0.Exit of the program.')
        if(a.isdigit()==True):
            n = int(a)
            if (int(a) == 1):
                print('New user registration.')
                reg_login = input('Create a username:')
                for i in range(len(data)):
                    str = data[i].split(" ")
                    if str[0] == reg_login:
                        j = 1
                        c = i
                        break
                if (reg_login == str[0]):
                    print('User exists!')
                else:
                    reg_password = input('Create a password:')
                    print('You successfully registred!')
                    data.append(reg_login + " " + reg_password)
            elif (int(a) == 2):
                j = 0
                print('Login to the system.')
                login = input('Enter username:')
                for i in range(len(data)):
                    str = data[i].split(" ")
                    if (str[0] == login):
                        j = 1
                        c = i
                        break
                if j != 1:
                    print('Wrong username!')
                else:
                    password = input('Enter password:')
                    if (password == str[1]):
                        print('You are logged in as a user!')
                        while ((int(b) != 3) or (int(b) != 4)):
                            b = input(
                                'Select:\n1.Change login.\n2.Change password.\n3.Exit of the account.\n4.Exit of the program.')
                            if (b.isdigit() == True):
                                if (int(b) == 1):
                                    for i in range(len(data)):
                                        str = data[i].split(" ")
                                        if str[0] == login:
                                            c = i
                                            break
                                    data.pop(c)
                                    login = input('Create a new username:')
                                    password1 = 0
                                    while (password1 != str[1]):
                                        password1 = input('Enter password:')
                                        if (password1 == str[1]):
                                            data.append(login + " " + str[1])
                                        else:
                                            print('Wrong password!')
                                elif (int(b) == 2):
                                    for i in range(len(data)):
                                        str = data[i].split(" ")
                                        if str[0] == login:
                                            j = 1
                                            c = i
                                            break
                                    data.pop(c)
                                    password1 = 0
                                    while (password1 != str[1]):
                                        password1 = input('Enter password:')
                                        if (password1 == str[1]):
                                            password = input('Create a new password:')
                                            data.append(str[0] + " " + password)
                                        else:
                                            print('Wrong password!')
                                elif (int(b) == 3):
                                    break
                                elif (int(b) == 4):
                                    return 0
                                else:
                                    print('Wrong input!')
                            else:
                                print('Wrong input!')
                    else:
                        print('Wrong password!')
            else:
                print('Wrong input!')
        else:
            print('Wrong input!')
main1()