Suresh is a fan of Mario. But being a programmer he decided to modify the game. In one module he needs your help. He modified the game as follows:
There are N stones on the way indexed from 1 to N. Every stone having index i is associated with points equal to i^th fibonacci number. That is if there are 5 stones then the points associated with them are 1,1,2,3,5 resp.
Mario gets the point when he steps on the stone. But during the gameplay he steps on M stones and skips some. So you have to find the sum of points Mario missed.

Input fomat:
First line contains 2 space separated integers containing N and M.
Next line contains M space separated integers denoting the points that he obtained.

Output format:
Print the sum of points Mario missed.

Constraints:
3 ≤ N ≤ 80
1 ≤ M ≤ N

SAMPLE INPUT
5 3
1 1 5

SAMPLE OUTPUT
5

Explanation

He missed the stones having points 2 and 3.
def missingSum(n, nos):
	inos = []
	inosSum = 0
	for k in nos:
		inos.append(int(k))
		inosSum += int(k)
	
	point1 = 1
	point2 = 1
	su = 2
	count = 3
	k  = 0
	
	while(count <= n ):
		su += point1 + point2
		k  = point1 + point2
		point1 = point2
		point2 = k
		count += 1
	
	print su - inosSum	
	
n,m = raw_input().split(" ")
n, m = int(n), int(m)
missingSum(n, raw_input().split(" "))