#define _CRT_SECURE_NO_WARNINGS

#pragma comment(linker, "/STACK:268435456")

#include <algorithm>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <vector>

using namespace std;

//const string IN_NAME = "input.txt";
//const string OUT_NAME = "output.txt";

const string NAME = "circle";
const string IN_NAME = NAME + ".in";
const string OUT_NAME = NAME + ".out";

template<class T> T abs(T &x)       { return ((x) >= 0) ? (x) : (-(x)); }
template<class T> T sqr(T &x)       { return (x) * (x); }
//template<class T> T min(T a, T b) { return ((a) < (b)) ? (a) : (b); }
//template<class T> T max(T a, T b) { return ((a) > (b)) ? (a) : (b); }

#define sz(v) ((int) ((v).size()))
#define all(v) (v).begin(), (v).end()
#define fori(i,n) for (int i = 0; i < (n); i++)

typedef long long ll;
typedef pair<int, int> ii;

//------------------------------------------------------------------------------

const ll INF = 1000LL*1000LL*1000LL*1000LL*1000LL*1000LL;

int n, k;
bool a[2005002];

deque<bool> fd, bd;

int main()
{
    freopen(IN_NAME.c_str(), "r", stdin);
    freopen(OUT_NAME.c_str(), "w", stdout);

    scanf("%d%d", &n, &k);
    for (int i = 0; i < k; i++)
    {
        int x;
        scanf("%d", &x);
        x--;
        a[x] = true;
    }

    ll s = 0;
    for (int i = 0; i < 2*n; i++)
    {
        if (a[i])
        {
            if (0 <= i && i < n)
                s += i;
            else
                s += 2*n - i;
        }
    }

    ll best = s;

    for (int i = 1; i <= n; i++)
        fd.push_back(a[i]);
    for (int i = n+1; i < 2*n; i++)
        bd.push_back(a[i]);
    bd.push_back(a[0]);

    ll fTrue = 0;
    ll bTrue = 0;
    for (int i = 0; i < sz(fd); i++)
        if (fd[i]) fTrue++;
    for (int i = 0; i < sz(bd); i++)
        if (bd[i]) bTrue++;

    for (int i = 1; i < 2*n; i++)
    {
        s -= fTrue;
        s += bTrue;

        if (s < 0)
        {
            throw 0;
        }

        if (s < best)
        {
            best = s;
        }

        fd.push_back(bd.front());
        bd.pop_front();
        if (fd.back())
        {
            fTrue++;
            bTrue--;
        }

        bd.push_back(fd.front());
        fd.pop_front();
        if (bd.back())
        {
            fTrue--;
            bTrue++;
        }
    }

    cout << best;
}