Logical Strategy: Casework

In a version of BASIC programming language, the name of a variable is a string of either one or two alphanumeric characters (An alphanumeric characters is either one of the 26 English letters or one of the 10 digits) starting with a letter. If uppercase and lowercase letters are not distinguishable and five two-character strings are reserved for another purpose, how many variables can be used in this version of BASIC?