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.
a=map(int,raw_input().split())
x=a[0]
y=map(int,raw_input().split())
if(x>2):
	b=[1,1]
	for i in range(x-2):
		su=b[len(b)-1]+b[len(b)-2]
		b.append(su)
	print sum(b)-sum(y)
elif(x==2):
	print 2-sum(b)
else:
	print 1-sum(b)