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Write a function named valid_passwords that examines a list of passwords and password policies and returns how many of the passwords are acceptable by those policies. Your function accepts a string parameter representing a file name as a parameter. Each line of the file contains a policy followed by a password. A policy is of the form MIN-MAX LETTER indicating that the given letter must occur MIN to MAX number of times in the password, inclusive. For example, if the file named passwords.txt contains the following values:

1-3 a: abcde
1-3 b: cdefg
2-9 c: ccccccccc

For example, 1-3 a means that the password must contain a at least 1 time and at most 3 times. In the above example, 2 passwords are valid. The middle password, cdefg, is not it contains no instances of b, but needs at least 1. The first and third passwords are valid: they contain one a or nine c, both within the limits of their respective policies. So the call of valid_passwords("passwords.txt") should return 2.

You may assume that the file exists and is readable and that it follows the format described above.

Function: Write a Python function as described, not a complete program.

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