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 Python (3.4.3) (judge verdict: Wrong Answer)

N, M = (int(i) for i in input().split())
A = [1]*(N+1)
for i in range(M):
  a = int(input())
  A[a] = -1

ans = 0
dp=[[0 for i in range(N+1)] for j in range(N+1)]
#dp[何回目の移動か][今どこにいるか] = 何通りあるか
dp[0][0] = 1

for i in range(N):
  for j in range(N):
    if j+1 <= N:
      if A[j+1] == 1:
        dp[i+1][j+1] += dp[i][j]
    if j+2 <= N:
      if A[j+2] == 1:
        dp[i+1][j+2] += dp[i][j]

for i in range(N):
  ans += dp[i][N]

print(ans%1000000007)