MathNet 2011 Question 13

Let set $M = \{1, 2, 3, ..., 50\}$. Find all positive integer $n$, such that there are at least two different elements $a$ and $b$ in any subset with 35 elements of $M$, such that $a + b = n$ or $a - b = n$. (posed by Li Shenghong)