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 5.4.1) (judge verdict: Wrong Answer)

#include<stdio.h>

int fib(int n){
    if (n <= 2) return 1;
    else return fib(n - 1) + fib (n - 2);
}

int main(void){
    int n, m, a[100001], i, j = 0, count = 0, ans = 1;
    scanf("%d %d", &n, &m);
    for(i = 0; i < m; i++){
        scanf("%d", &a[i]);
        //printf("a[%d]=%d\n", i, a[i]);
    }
    for(i = 0; i <= n; i++){
        if(a[j] + 1 == a[j + 1]){
            ans = 0;
            break;
        } else if(i == a[j]){
            ans *= fib(count);
            //printf("ans = %d\n", ans);
            j++;
            count = 0;
        } else {
            count++;
        }
    }
    count--;
    ans *= fib(count);
    printf("%d\n", ans % 1000000007);
    return 0;
}