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++ (GCC 9.2.1) (judge verdict: Accepted)

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

int main() {
  int N, M;
  cin >> N >> M;
  vector<int64_t> stairs(N + 1);
  stairs.at(0) = 1;
  int64_t MOD = 1000000000 + 7;
  set<int> broken;
  for(int i = 0; i < M; i++) {
    int a;
    cin >> a;
    broken.insert(a);
  }
  for(int i = 1; i <= N; i++) {
    if(i == 1) {
      if(broken.count(1)) {
        stairs.at(1) = 0;
      }
      else {
        stairs.at(1) = 1;
      }
    }
    else {
      if(broken.count(i)) {
        stairs.at(i) = 0;
      }
      else {
        stairs.at(i) = stairs.at(i - 1) + stairs.at(i - 2);
        stairs.at(i) %= MOD;
      }
    }
  }
  cout << stairs.at(N) << endl;  
}