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)

#include <stdio.h>

#define MOD_BY 1000000007

int add(int a, int b) {
	return a + b - MOD_BY * (a + b >= MOD_BY);
}

int N, M;
int a[114514];

char dame[114514];

int memo[114514];

int kitaeri(int cur) {
	if (cur == N) return 1;
	if (cur > N) return 0;
	if (dame[cur]) return 0;
	if (memo[cur]) return ~memo[cur];
	return ~(memo[cur] = ~add(kitaeri(cur + 1), kitaeri(cur + 2)));
}

int main(void) {
	int i;
	if (scanf("%d%d", &N, &M) != 2) return 1;
	for (i = 0; i < M; i++) {
		if (scanf("%d", &a[i]) != 1) return 1;
		dame[a[i]] = 1;
	}
	printf("%d\n", kitaeri(0));
	return 0;
}