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: Runtime Error)

#include <iostream>
#include <vector>
#include <cmath>
#include <algorithm>
#include <functional>
#include <cassert>

typedef long long ll;

int main()
{
    int N, M;
    std::cin >> N >> M;

    //大丈夫なところはtrue
    std::vector<bool> a(N+1, true);
    for(int i=0; i<N; ++i)
    {
        int num;
        std::cin >> num;
        a[num] = false;
    }

    std::vector<ll> dp(N+1);
    for(int i=0; i<N; ++i)
    {
        for(int j=i+1; j<=std::min(i+2, N); ++j)
            if(a[j])
            {
                dp[j] = dp[j-1];
                dp[j]%=dp[j];
            }
    }

    std::cout << dp[N] << std::endl;

    return 0;
}