Words in “index of coincidence” in relation to the Vigenère cipher I am trying to interpret the formula for index of coincidence from here…


  


I am not understanding $f_i$. How can I calculate $f_i$ in this context? Is this the appearance of single word like (a) in the whole sentence or what ? I am trying to solve it manually without using the mentioned website tool, but I am getting the wrong answer.
Here $f_i$ is simply the number of times the character $i$ appears in the ciphertext of length $N$ and where $Z$ is the alphabet size. 

If you had ciphertext ADCXU ZMDYZ DXZUM and which was derived from English plaintext then $N=15$  and $f_A=1,f_B=0,f_C=1, f_D=3,\ldots, f_Z=2.$