#!/usr/bin/env python
# -*- coding: utf-8 -*-

import sys

a = sys.stdin.readline().strip()
leng, _ = map(int, a.split())

a = sys.stdin.readline().strip()
opors = map(int, a.split())

if len(opors) < 4:
    print leng
else:
    res = opors[1] + leng - opors[-2]
    for i in xrange(1, len(opors) - 2):
        right = hi = opors[i + 1]
        left = lo = opors[i]
        if left < (right - left):
            hi = left * 2
        if leng - right < (right - left):
            lo = 2*right - leng

        if hi < lo:
            res += right - left
        else:
            res += (right - hi)
            res += (lo - left)

    print res