Make Program That Counts The Frequency Of Each Letter In The Alphabet?

I'm trying to make this program that counts the frequency of each letter in the alphabet for a set of ciphertext, the calculation i want to do is:lettersum*(lettersum - 1) + lettersum*(lettersum - 1) for however many letters example: User puts aabbcc in the ciphertext box , the output should be
2(2-1)+2(2-1)+2(2-1) =6 If the user inputs aaaaab the output should be 5(5-1)+1(1-1) = 20 The code I have so far only counts the last number of letters for example: aabbcc only reads the cc part and only outputs 2(2-1) = 2 instead of 2(2-1) + 2(2-1) + 2(2-1) = 6.The code below is part of a button_Click item

[Code]...