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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 $f_A=1,f_B=0,f_C=1, f_D=3,\ldots, f_Z=2.$


You've been given the key length, plus half the characters in the key text. By the look of things, the ciphertext even contains word spaces and punctuation, which will help a lot. Start by decrypting the text using the characters you already know: > Peit Wokm! Mhfa fepatgb ets bvvrvxmea ebte bae twkd qiqi .a.i .a.i .a.i .a.i.a. i.a .i.a.i.a. i.a. i.a ...

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