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 fib(n):

    f=[]
    f.append(1)
    f.append(1)

    for i in range(2,n):
        p=f[i-1]
        q=f[i-2]
        f.append(p+q)

    #print f
    return sum(f)

temp=map(int,raw_input().split())
n=temp[0]
arr=map(int,raw_input().split())

a=[]
c1=fib(n)

print c1-sum(arr)