human
Alice and Bob are playing a game on a line with n cells. There are n cells labeled from 1 through n. For each i from 1 to n-1, cells i and i+1 are adjacent.

Alice initially has a token on some cell on the line, and Bob tries to guess where it is. 

Bob guesses a sequence of line cell numbers x_1, x_2, …, x_k in order. In the i-th question, Bob asks Alice if her token is currently on cell x_i. That is, Alice can answer either "YES" or "NO" to each Bob's question.

At most one time in this process, before or after answering a question, Alice is allowed to move her token from her current cell to some adjacent cell. Alice acted in such a way that she was able to answer "NO" to all of Bob's questions.

Note that Alice can even move her token before answering the first question or after answering the last question. Alice can also choose to not move at all.

You are given n and Bob's questions x_1, …, x_k. You would like to count the number of scenarios that let Alice answer "NO" to all of Bob's questions. 

Let (a,b) denote a scenario where Alice starts at cell a and ends at cell b. Two scenarios (a_i, b_i) and (a_j, b_j) are different if a_i ≠ a_j or b_i ≠ b_j.

Input

The first line contains two integers n and k (1 ≤ n,k ≤ 10^5) — the number of cells and the number of questions Bob asked.

The second line contains k integers x_1, x_2, …, x_k (1 ≤ x_i ≤ n) — Bob's questions.

Output

Print a single integer, the number of scenarios that let Alice answer "NO" to all of Bob's questions.

Examples

Input

5 3
5 1 4

Output

9

Input

4 8
1 2 3 4 4 3 2 1

Output

0

Input

100000 1
42

Output

299997

Note

The notation (i,j) denotes a scenario where Alice starts at cell i and ends at cell j.

In the first example, the valid scenarios are (1, 2), (2, 1), (2, 2), (2, 3), (3, 2), (3, 3), (3, 4), (4, 3), (4, 5). For example, (3,4) is valid since Alice can start at cell 3, stay there for the first three questions, then move to cell 4 after the last question. 

(4,5) is valid since Alice can start at cell 4,...
assistant
#Here is Python code to print a single integer, the number of scenarios that let Alice answer "NO" to all of Bob's questions.

import sys
# import bisect
# from collections import deque

# from types import GeneratorType
# def bootstrap(func, stack=[]):
#     def wrapped_function(*args, **kwargs):
#         if stack:
#             return func(*args, **kwargs)
#         else:
#             call = func(*args, **kwargs)
#             while True:
#                 if type(call) is GeneratorType:
#                     stack.append(call)
#                     call = next(call)
#                 else:
#                     stack.pop()
#                     if not stack:
#                         break
#                     call = stack[-1].send(call)
#             return call
#     return wrapped_function

Ri = lambda : [int(x) for x in sys.stdin.readline().split()]
ri = lambda : sys.stdin.readline().strip()
 
def input(): return sys.stdin.readline().strip()
def list2d(a, b, c): return [[c] * b for i in range(a)]
def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]
def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]
def ceil(x, y=1): return int(-(-x // y))

def Yes(): print('Yes')
def No(): print('No')
def YES(): print('YES')
def NO(): print('NO')

n,k = Ri()
a = Ri()

ans = 0
s = set()
dicinc = {}
dicdec = {}
for i in range(k-1,-1,-1):
    if a[i]+1 <= n :
        if a[i]+1 not in s:
            dicinc[a[i]] = True
        else:
            dicinc[a[i]] = False
    if a[i]-1 >= 1:
        if a[i]-1 not in s:
            dicdec[a[i]] = True
        else:
            dicdec[a[i]] = False
        # ans+=1
    s.add(a[i])
for i in dicinc:
    if dicinc[i] >= 0:
        ans+=dicinc[i]
for i in dicdec:
    if dicdec[i] >= 0:
        ans+=dicdec[i]

rem = n- len(s)
if 1 not in s:
    ans-=1
if n not in s:
    ans-=1
ans+= rem*3
# print(rem)
print(ans)