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: Accepted)

import sys
 
sys.setrecursionlimit(10 ** 6)
INF = float("inf")
MOD = 10 ** 9 + 7
 
 
def input():
    return sys.stdin.readline().strip()
 
 
def main():
    N, M = map(int, input().split())
    dp = [0] * (N + 1)
    safe = [True] * (N + 1)
    for _ in range(M):
        a = int(input())
        safe[a] = False
 
    dp[0] = 1
    for i in range(1, N + 1):
        if i == 1 and safe[i]:
            dp[i] = dp[i - 1]
        else:
            if safe[i]:
                dp[i] = dp[i - 1] + dp[i - 2]
                dp[i] %= MOD
 
    print(dp[-1])
 
 
if __name__ == "__main__":
    main()