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 PyPy3 (2.4.0) (judge verdict: Runtime Error)

"""
N段の階段 M個の階段が壊れている
"""

N, M = map(int, input().split())
arr = [int(input()) for _ in range(M)]
no_step =[0]*N

for i in range(M):
    no_step[arr[i]-1]=1

count = [0]*N
if no_step[0]==0:
    count[0]=1

if no_step[1]==1:
    count[1]=0
elif no_step[0]==0 and no_step[1]==0:
    count[1]=2
elif no_step[0]==1 and no_step[1]==0:
    count[1]=2

for i in range(3,N+1,1):
    count[i-1] = count[i-2]+count[i-3]
    if no_step[i-1] == 1:
        count[i-1] = 0

print(count[-1]%1000000007)