Problems with unity pro extract one character from string


I have tag data type is String consist of 22 characters, i want to extract one character and convert into Int to do some calculations, i have used( MID_INT) Function Block extracts a sub-string of characters starting from a certain rank. The result is a character string but I want to put in Array of strings or Array of Bytes and then take every Byte or character to do some calculations.
how I can extract one character from string[n] composed of set of characters (n) is number of characters
Is There any function Block can i use it?