MathNet 2007-part-01 — 187 problems (EN–VI) Question 115

Find all integers $n \ge 3$ such that: for any two positive integers $m \lt n-1$, $r \lt n-1$ there exist $m$ distinct elements of the set $\{1, 2, \dots, n-1\}$ whose sum is congruent to $r$ modulo $n$.