Junior Balkan Mathematical Olympiad 2021 - Problem 4 - 1 July 2021

Problem 4. Let $M$ be a subset of the set of $2021$ integers $\{1, 2, 3, ..., 2021\}$ such that for any three elements (not necessarily distinct) $a, b, c$ of $M$ we have $|a + b - c | > 10$ .
Determine the largest possible number of elements of $M$ .