Please use c# to solve the problem, do not use regular expressions and regex functions

You are given two strings S1 and S2 that can contain any ASCII symbols, except for the line feed character. Normalize the strings: delete all symbols that are not latin letters, digits or a whitespace character, and decapitalize every letter (make it lowercase). You should then delete all words which length is less or equal to 3 and all extra whitespace characters (so that there's no more than one whitespace character between two words). For the resulting strings count the following value: c/(S1+S2c), where c is the number of matching symbols between the strings S1 and S2 and Sn is the length of the n-th string