Problem:
Score : 300 points

Problem Statement

There is a staircase with N steps. Takahashi is now standing at the foot of the stairs, that is, on the 0-th step.
He can climb up one or two steps at a time.

However, the treads of the a_1-th, a_2-th, a_3-th, \ldots, a_M-th steps are broken, so it is dangerous to set foot on those steps.

How many are there to climb up to the top step, that is, the N-th step, without setting foot on the broken steps?
Find the count modulo 1\ 000\ 000\ 007.

Constraints

1 \leq N \leq 10^5

0 \leq M \leq N-1

1 \leq a_1 < a_2 < ... < a_M \leq N-1

Input

Input is given from Standard Input in the following format:

N M
a_1
a_2
.
.
.
a_M

Output

Print the number of ways to climb up the stairs under the condition, modulo 1\ 000\ 000\ 007.

Sample Input 1

6 1
3

Sample Output 1

4

There are four ways to climb up the stairs, as follows:

0 \to 1 \to 2 \to 4 \to 5 \to 6

0 \to 1 \to 2 \to 4 \to 6

0 \to 2 \to 4 \to 5 \to 6

0 \to 2 \to 4 \to 6

Sample Input 2

10 2
4
5

Sample Output 2

0

There may be no way to climb up the stairs without setting foot on the broken steps.

Sample Input 3

100 5
1
23
45
67
89

Sample Output 3

608200469

Be sure to print the count modulo 1\ 000\ 000\ 007.

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;

long long MOD = 1000000007;
int main() {
  int n, m;
  cin>>n>>m;
  int a[m];
  for (int i=0; i<m; i++) cin>>a[i];

  long long ans[n+1];
  int idx_a = 0;
  ans[0] = 1;
  if (a[0] == 1) {
    ++idx_a;
    ans[1] = 0;
  } else {
    ans[1] = 1;
  }
  for (int i=2; i<=n; ++i) {
    if (idx_a >= m) {
      ans[i] = (ans[i-1] + ans[i-2]) % MOD;
      continue;
    }
    if (a[idx_a] == i) {
      ans[i] = 0;
      ++idx_a;
      continue;
    }
    ans[i] = (ans[i-1] + ans[i-2]) % MOD;
  }
  cout<<ans[n]<<endl;
}