#include<bits/stdc++.h>
using namespace std;
const int maxn = 2e5 + 5;
long long n, dp[maxn][2], tdp[maxn][2];
int m;
vector<long long> pos, ps;
map<long long, int> id, tmp;
set<long long> s;
long long Solve(long long n, int len) {
// cerr << "Solve(" << n << ", " << len << ")" << endl;
// for(auto x : pos)
//     cerr << x << " ";
// cerr << endl;
// for(int i = 0; i < len; ++ i)
//     cerr << dp[i][0] << " ";
// cerr << endl;
// for(int i = 0; i < len; ++ i)
//     cerr << dp[i][1] << " ";
// cerr << endl;
    for(int l = 0, r = len - 1; l < r; ++ l, -- r) {
        for(int i = 0; i < 2; ++ i) {
            tdp[l][i] = 2e18;
            for(int j = 0; j < 2; ++ j) {
                for(int k = 0; k < 2; ++ k) {
                    tdp[l][i] = min(tdp[l][i], dp[l][j] + dp[r][k] + (i != j) + (i != k));
                }
            }
        }
    }
    if(n == 2) {
// cerr << tdp[0][0] << " " << tdp[0][1] << endl;
        return min(tdp[0][0], tdp[0][1]);
    }
    long long mid = n / 2;
    ps.clear(), tmp.clear();
    for(int l = 0; l < len / 2; ++ l) {
        assert(pos[l] <= mid);
        ps.push_back(pos[l]);
        ps.push_back(mid + 1 - pos[l]);
    }
    sort(ps.begin(), ps.end());
    ps.erase(unique(ps.begin(), ps.end()), ps.end());
    int nl = ps.size();
    for(int i = 0; i < nl; ++ i) {
        tmp[ps[i]] = i;
        if(id.count(ps[i])) {
            dp[i][0] = tdp[id[ps[i]]][0];
            dp[i][1] = tdp[id[ps[i]]][1];
        }
        else {
            dp[i][0] = 0;
            dp[i][1] = 1;
        }
    }
    swap(id, tmp);
    swap(pos, ps);
    return Solve(n / 2, nl);
}
mt19937_64 rnd(random_device{}());
int main() {
    scanf("%lld%d", &n, &m);
    // n = 1ll << 59;
    // m = 1e5;
    for(int i = 0; i < m; ++ i) {
        long long x;
        scanf("%lld", &x);
        // x = rnd() % n + 1;
        s.insert(x);
        pos.push_back(x);
        pos.push_back(n + 1 - x);
    }
    sort(pos.begin(), pos.end());
    pos.erase(unique(pos.begin(), pos.end()), pos.end());
    int len = pos.size();
    for(int i = 0; i < len; ++ i) {
        int v = s.count(pos[i]);
        id[pos[i]] = i;
        dp[i][v] = 0;
        dp[i][v ^ 1] = 1;
    }
    printf("%lld\n", Solve(n, len));
}