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: Accepted)

/*2019/08/25 */
#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<string.h>
#define MOD 1000000007
#define MIN 999999999
#define MAX -999999999
typedef long long int ll;
typedef unsigned long long int ull;
void llswap(ll *x,ll *y){ll temp;temp=*x;*x=*y;*y=temp;}
void swap(int *x,int *y){int tmp;tmp=*x;*x=*y;*y=tmp;}
int rmax(int x,int y){return x>y?x:y;}
int rmin(int x,int y){return x>y?y:x;}
ll llrmax(ll x,ll y){return x>y?x:y;}
ll llrmin(ll x,ll y){return x>y?y:x;}
int asc(const void *a,const void *b){return *(int*)a-*(int*)b;}
int desc(const void *a,const void *b){return *(int*)b-*(int*)a;}
int llasc(const void *a,const void *b){return *(ll*)a-*(ll*)b;}
int lldesc(const void *a,const void *b){return *(ll*)b-*(ll*)a;}
/*
int asc_t(const void *a,const void *b){return ((struct_t*)a)->member - ((struct_t*)b)->member;}
int desc_t(const void *a,const void *b){return ((struct_t*)b)->member - ((struct_t*)a)->member;}
*/
int main(){
    int n,m;
    static int stair[200000]={},a[200000];
    static ull dp[200000]={};

    scanf("%d %d",&n,&m);
    for(int i=0;i<m;i++){
        scanf("%d",&a[i]);
        stair[a[i]-1]=1;
    }
    dp[0]=1;
    if(stair[0]!=1){
        dp[1]=1;
    }
    for(int i=1;i<n;i++){
        if(stair[i]!=1){
            dp[i+1]=dp[i]+dp[i-1];
        }
        if(dp[i+1]>=MOD){
            dp[i+1]%=MOD;
        }
    }
    printf("%llu\n",dp[n]%MOD);
    return 0;
}