Unable to understand the editorial

Правка en2, от skpro19, 2017-08-22 14:13:09

The question is this-> 757C

I am unable to understand anything in the editorial, as well as the given code. Can someone please help?

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en2 Английский skpro19 2017-08-22 14:13:09 1961
en1 Английский skpro19 2017-08-22 13:49:32 2188 Initial revision (published)