#define _USE_MATH_DEFINES
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <cfloat>
#include <climits>
#include <cstring>
#include <cmath>
#include <fstream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<ll, int> ll_i;
typedef pair<double, int> d_i;
typedef pair<ll, ll> ll_ll;
typedef pair<double, double> d_d;
struct edge { int v, w; };

ll MOD = 1000000007;
ll _MOD = 1000000009;
double EPS = 1e-10;

int f(int x, int y) {
    if (x >= y) return 0;
    int z = (x + y) / 2 - x;
    return max(0, y - z * 2 - 1);
}

int main() {
    int n, m;
    while (cin >> n >> m) {
        int l = INT_MAX, r = INT_MIN;
        while (m--) {
            int x; cin >> x;
            l = min(l, x - 1);
            r = max(r, x);
        }
        if (l == 0 && r == n) {
            cout << 0 << endl;
            continue;
        }
        if (l > 0 && r < n) {
            cout << f(l, n - r) + f(n - r, l) << endl;
            continue;
        }
        int x;
        if (l == 0) x = n - r;
        if (r == n) x = l;
        ll sum = 0;
        for (int y = 1; y <= x - 1; y++)
            sum += f(y, x - y);
        cout << sum << endl;
    }
}