CF447A.DZY Loves Hash

DZY has a hash table with $p$ buckets, numbered from $0$ to $p-1$ . He wants to insert $n$ numbers, in the order they are given, into the hash table. For the $i$ -th number $x_{i}$ , DZY will put it into the bucket numbered $h(x_{i})$ , where $h(x)$ is the hash function. In this problem we will assume, that $h(x)=x mod p$ . Operation $a mod b$ denotes taking a remainder after division $a$ by $b$ .

However, each bucket can contain no more than one element. If DZY wants to insert an number into a bucket which is already filled, we say a "conflict" happens. Suppose the first conflict happens right after the $i$ -th insertion, you should output $i$ . If no conflict happens, just output -1.

The first line contains two integers, $p$ and $n$ $(2<=p,n<=300)$ . Then $n$ lines follow. The $i$ -th of them contains an integer $x_{i}$ $(0<=x_{i}<=10^{9})$ .

Output a single integer — the answer to the problem.