NL-NWO 1974 Problem 2

$n>2$ numbers, $x_1, x_2, ..., x_n$ are odd . Prove that $4$ divides $x_1x_2+x_2x_3+...+x_{n-1}x_n+x_nx_1 -n.$

Dataset text AI-MO AoPS. Quoted with source.

Solutions

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