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: Runtime Error)

n,m = list(map(int, input().split()))
s = [0 for i in range(n+1)]
t = [-1 for i in range(n+1)]
for i in range(m):
    x = int(input())
    s[x] = 1

def dx(r):
    global t
    if t[r]>=0:
        return t[r]
    x=0
    y=0
    #print(i)
    if s[r+1]==0 and r==n-2:
        return 2
    if s[r+1]==1 and r==n-2:
        return 1
    if r==n-1:
        return 1
    if r==n:
        return 0
    if s[r+1] == 0:
        x=dx(r+1)
    if s[r+2] == 0:
        y=dx(r+2)
    #print([i,x,y])
    t[r]=(x+y)%1000000007
    return (x+y)%1000000007

ans=dx(0)

print(ans)