fr = open('accounts.txt', 'r')
while True:
username = input('Enter your username: ')  # Ask for their username

    if username in fr.read():  # Check if username exists
        password = input('Enter password: ')  # Ask for password if username exists

        if username+password in fr.read():
            print('Welcome ' + username)
            break
        else:
            print('Wrong password')