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 main(void){
int n,m;
  long y=1000000007;
  scanf("%d%d",&n,&m);
  int d[m];
  for(int i=0;i<m;i++){
	scanf("%d",&d[i]);
  }
  for(int i=0;i<m-1;i++){
	if(d[i+1]-d[i]==1){
      printf("0");
      return 0;
    }
  }
  
long x[n],s=0;
  x[0]=1;
  x[1]=2;
  if(d[0]==1||d[0]==2){
    s++;
	x[0]=0;
    x[1]=1;
  }
 
  for(int i=2;i<n;i++){
	x[i]=0;
  }
  for(int i=2;i<n;i++){
    //printf("%d段目 ",i+1);
    if(i+1==d[s]){s++;
      //printf("skip\n");
      continue;
    }
    x[i]=(x[i-2]+x[i-1]);
    //printf("%ld\n",x[i]);
  }
  printf("%ld",x[n-1]%y);
return 0;
}