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: Wrong Answer)

#include <bits/stdc++.h>

using namespace std;

int main() {
    cin.tie(nullptr); ios::sync_with_stdio(false);

    int N,M;cin>>N>>M;
    const long MOD = 1e9+7;
    auto mod = [&](long x) {
        return x % MOD;
    };
    vector<long> dp(3, 0);
    dp[0] = 1;
    int a; cin>>a;
    for (int i = 0; i < N; i++) {
        if (i == a) {
            if (M) {
                cin>>a;
                M--;
            }
            dp[i%3] = 0;
        }
        dp[(i+1)%3] = mod(dp[(i+1)%3] + dp[i%3]);
        dp[(i+2)%3] = dp[i%3];
        cerr << dp[i%3] << endl;
    }
    auto ans = dp[N%3];
    cout << ans << endl;

    return 0;
}