# Algebraic differential cryptanalysis

I have been studying on algebraic methods on cryptanalysis of block ciphers. This is where I am reading from currently

I need some help to understand Attack C.

Excluding equations from the first r rounds till which the differential characteristic holds, we are just left with the SBox equations and one constraint from the input difference to the $$(r+1)^{th}$$ round S Boxes as a consequence of the r round differential, for each of the plaintext pairs.

I cannot understand how they are considering an equivalent structure with 2R -1 rounds from C' to C''.

A brief explanation would be really helpful.