Broken Steps#8235
In how many ways can you reach the th step if you can move to the next step, skip one step, or skip two steps, given that some steps are broken?
The first line contains two integers: , the step number you want to reach, and , the number of broken steps. 1 \leq K \leq N \leq 60.The second line lists the numbers of the broken steps.
Output a single integer representing the number of ways to reach the th step, or if it is impossible to reach that step.
Example 1