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, stay there for the first question, the move to cell 5 for the next two questions. Note that (4,5) is only counted once, even though there are different questions that Alice can choose to do the move, but remember, we only count each pair of starting and ending positions once.

In the second example, Alice has no valid scenarios.

In the last example, all (i,j) where |i-j| ≤ 1 except for (42, 42) are valid scenarios.
#!/usr/bin/env python
"""<https://github.com/cheran-senthil/PyRival>"""
from __future__ import division, print_function

import os
import sys
from io import BytesIO, IOBase

if sys.version_info[0] < 3:
    from __builtin__ import xrange as range
    from future_builtins import ascii, filter, hex, map, oct, zip
else:
    _str = str
    str = lambda x=b"": x if type(x) is bytes else _str(x).encode()


def main():
    n, k = readlist()
    x = readlist()

    counter = [(-1, -1)] * (n + 1)
    for i, xi in enumerate(x):
        if counter[xi][0] == -1:
            counter[xi] = (i, -1)
        else:
            counter[xi] = (counter[xi][0], i)

    res = 0
    for i in range(1, n + 1):
        if counter[i][0] == -1:
            res += 1

    for i in range(1, n):
        if counter[i][0] != -1:
            if counter[i][1] != -1:
                if counter[i + 1][0] == -1 or counter[i + 1][0] > counter[i][1]:
                    res += 1
            else:
                if counter[i + 1][0] == -1 or counter[i + 1][0] > counter[i][0]:
                    res += 1
        else:
            res += 1

    for i in range(1, n):
        if counter[i + 1][0] != -1:
            if counter[i + 1][1] != -1:
                if counter[i][0] == -1 or counter[i][0] > counter[i + 1][1]:
                    res += 1
            else:
                if counter[i][0] == -1 or counter[i][0] > counter[i + 1][0]:
                    res += 1
        else:
            res += 1

    print(res)


# region template

BUFSIZE = 8192


class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._buffer = BytesIO()
        self._fd = file.fileno()
        self._writable = "x" in file.mode or "r" not in file.mode
        self.write = self._buffer.write if self._writable else None

    def read(self):
        if self._buffer.tell():
            return self._buffer.read()
        return os.read(self._fd, os.fstat(self._fd).st_size)

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self._buffer.tell()
            self._buffer.seek(0, 2), self._buffer.write(b), self._buffer.seek(ptr)
        self.newlines -= 1
        return self._buffer.readline()

    def flush(self):
        if self._writable:
            os.write(self._fd, self._buffer.getvalue())
            self._buffer.truncate(0), self._buffer.seek(0)


class ostream:
    def __lshift__(self, a):
        if a is endl:
            sys.stdout.write(b"\n")
            sys.stdout.flush()
        else:
            sys.stdout.write(str(a))
        return self


def print(*args, **kwargs):
    """Prints the values to a stream, or to sys.stdout by default."""
    sep, file = kwargs.pop("sep", b" "), kwargs.pop("file", sys.stdout)
    at_start = True
    for x in args:
        if not at_start:
            file.write(sep)
        file.write(str(x))
        at_start = False
    file.write(kwargs.pop("end", b"\n"))
    if kwargs.pop("flush", False):
        file.flush()


sys.stdin, sys.stdout = FastIO(sys.stdin), FastIO(sys.stdout)
cout, endl = ostream(), object()

readline = sys.stdin.readline
readlist = lambda var=int: [var(n) for n in readline().split()]
input = lambda: readline().rstrip(b"\r\n")

# endregion

if __name__ == "__main__":
    main()
