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.
n, m = [int(x) for x in raw_input().split(" ")]
fibo=[1]
a=0
b=1
for i in range(0, n-1)	:
	c=a+b
	fibo.append(c)
	a=b
	b=c

stepped=[int(x) for x in raw_input().split(' ')]

sums=0
for s in stepped :
	sums=sums+s

sumf=0
for i in fibo :
	sumf=sumf+i

printing=sumf-sums
print printing