Broken Steps

In how many ways can you reach the Nth step if you can move to the next step, skip one step, or skip two steps, given that some steps are broken?

Input

The first line contains two integers: N, the step number you want to reach, and K, the number of broken steps. (1 ≤ K ≤ N ≤ 60).The second line lists the numbers of the broken steps.

Output

Output a single integer representing the number of ways to reach the Nth step, or -1 if it is impossible to reach that step.

Examples