// #pragma GCC target("avx2")
// #pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")
#include <algorithm>
#include <bitset>
#include <cassert>
#include <climits>
#include <cmath>
#include <complex>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>
using namespace std;
using lg = long long;
using pii = pair<int, int>;
using pll = pair<lg, lg>;
#define TEST cerr << "TEST" << endl
#define AMARI 998244353
// #define AMARI 1000000007
#define TEMOTO ((sizeof(long double) == 16) ? false : true)
#define TIME_LIMIT 1980 * (TEMOTO ? 1 : 1000)
#define el '\n'
#define El '\n'

lg func(lg a){
    if(a % 2 == 1 || a <= 3)return 0LL;
    return func(a / 2) * 2LL + 1LL;
}

#define MULTI_TEST_CASE false
void solve(void) {
    int n,m;
    cin >> n >> m;
    vector<lg> a(m);
    for(int i = 0; i < m; i++){
        cin >> a[i];
    }
    lg ans = n - m;
    for(int i = 0; i < m - 1; i++){
        ans -= func(a[i + 1] - a[i]);
    }
    lg hajidif1 = a[0] - 1,hajidif2 = n - a.back();
    if(hajidif1 >= 2){
        ans--;
        ans -= func(hajidif1 - 1);
    }
    if(hajidif2 >= 2){
        ans--;
        ans -= func(hajidif2 - 1);
    }
    cout << ans << el;
    return;
}

void calc(void) {
    return;
}

int main(void) {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    calc();
    int t = 1;
    if(MULTI_TEST_CASE) cin >> t;
    while(t--) {
        solve();
    }
    return 0;
}