CF362B.Petya and Staircases

Little boy Petya loves stairs very much. But he is bored from simple going up and down them — he loves jumping over several stairs at a time. As he stands on some stair, he can either jump to the next one or jump over one or two stairs at a time. But some stairs are too dirty and Petya doesn't want to step on them.

Now Petya is on the first stair of the staircase, consisting of n stairs. He also knows the numbers of the dirty stairs of this staircase. Help Petya find out if he can jump through the entire staircase and reach the last stair number n without touching a dirty stair once.

One has to note that anyway Petya should step on the first and last stairs, so if the first or the last stair is dirty, then Petya cannot choose a path with clean steps only.

小男生佩佳非常喜爱楼梯。但他厌倦了简单地逐级上下楼梯——他喜欢一次跳过若干级楼梯。当他站在某一级楼梯上时，他可以跳到下一级楼梯，也可以一次跳过一级或两级楼梯。但有些楼梯太脏了，佩佳不想踩上去。

现在佩佳站在一个由 $n$ 级楼梯组成的楼梯的第一级上。他还知道这个楼梯中所有脏楼梯的编号。请帮助佩佳判断：他能否跳过整个楼梯，并最终到达第 $n$ 级楼梯，且在此过程中完全不踩任何脏楼梯。

需要注意的是，佩佳必须踩在第一级和最后一级楼梯上，因此如果第一级或最后一级楼梯是脏的，则佩佳无法找到一条仅由干净楼梯构成的路径。

The first line contains two integers n and m (1 ≤ n ≤ 109, 0 ≤ m ≤ 3000) — the number of stairs in the staircase and the number of dirty stairs, correspondingly. The second line contains m different space-separated integers _d_1, _d_2, ..., d__m (1 ≤ d__i ≤ n) — the numbers of the dirty stairs (in an arbitrary order).

第一行包含两个整数 $n$ 和 $m$（$1 \leq n \leq 10^9$，$0 \leq m \leq 3000$），分别表示楼梯的总阶数和脏楼梯的阶数。
第二行包含 $m$ 个互不相同的、以空格分隔的整数 $d_1,\,d_2,\,\dots,\,d_m$（$1 \leq d_i \leq n$），表示脏楼梯的编号（顺序任意）。

Print "YES" if Petya can reach stair number n, stepping only on the clean stairs. Otherwise print "NO".

如果Petya能够仅踩在干净的台阶上到达第 $n$ 级台阶，则输出 "YES"；否则输出 "NO"。