$n \ge 4$ real numbers are arranged in a circle. It turned out that for any four consecutive numbers $a, b, c, d$, that lie on the circle in this order, holds $a+d = b+c$. For which $n$ does it follow that all numbers on the circle are equal? Proposed by Oleksiy Masalitin
Problem
Source: Ukrainian Mathematical Olympiad 2023. Day 1, Problem 9.1
Tags: combinatorics, equality