// author: hotman78
// date: 2023/09/29-21:37:13

// --- begin raw code -----------------
// #include"cpplib/util/template.hpp"
// 
// int main(){
//     lint n,m;
//     cin>>n>>m;
//     vec a(m);
//     rep(i,m)cin>>a[i];
//     lint ans=0;
//     rep(i,m-1){
//         if(a[i+1]-a[i]>1){
//             lint sz=1;
//             auto f=[&](auto f,lint x)->lint
//             {
//                 if(x==1)return 1;
//                 if(x%2)return f(f,x/2)*2;
//                 else return x;
//             };
//             ans+=f(f,a[i+1]-a[i]-1);
//         }
//     }
//     cout<<ans<<endl;
// }
// --- end raw code -----------------

#line 2 "cpplib/util/template.hpp"
#ifdef LOCAL
#define _GLIBCXX_DEBUG
#endif
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx2")
#include <bits/stdc++.h>
using namespace std;
#line 1 "cpplib/util/ioutil.hpp"
// template <class Head,class... Args>
// std::ostream& output(std::ostream& out,const Head& head,const Args&... args){
//     out>>head;
//     return output(head,args...);
// }
// template <class Head>
// std::ostream& output(std::ostream& out,const Head& head){
//     out>>head;
//     return out;
// }

template <typename T, typename E>
std::ostream &operator<<(std::ostream &out, std::pair<T, E> v) {
    out << "(" << v.first << "," << v.second << ")";
    return out;
}

// template <class... Args>
// ostream& operator<<(ostream& out,std::tuple<Args...>v){
//     std::apply(output,v);
//     return out;
// }
#line 11 "cpplib/util/template.hpp"
struct __INIT__ {
    __INIT__() {
        cin.tie(0);
        ios::sync_with_stdio(false);
        cout << fixed << setprecision(15);
    }
} __INIT__;
typedef long long lint;
constexpr long long INF = 1LL << 60;
constexpr int IINF = 1 << 30;
constexpr double EPS = 1e-10;
#ifndef REACTIVE
#define endl '\n';
#endif
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
typedef vector<vector<vector<lint>>> mat3;
typedef vector<string> svec;
typedef vector<vector<string>> smat;
template <typename T> using V = vector<T>;
template <typename T> using VV = V<V<T>>;
template <typename T> inline void output(T t) {
    bool f = 0;
    for (auto i : t) {
        cout << (f ? " " : "") << i;
        f = 1;
    }
    cout << endl;
}
template <typename T> inline void output2(T t) {
    for (auto i : t)
        output(i);
}
template <typename T> inline void debug(T t) {
    bool f = 0;
    for (auto i : t) {
        cerr << (f ? " " : "") << i;
        f = 1;
    }
    cerr << endl;
}
template <typename T> inline void debug2(T t) {
    for (auto i : t)
        debug(i);
}
#define loop(n) for (long long _ = 0; _ < (long long)(n); ++_)
#define _overload4(_1, _2, _3, _4, name, ...) name
#define __rep(i, a) repi(i, 0, a, 1)
#define _rep(i, a, b) repi(i, a, b, 1)
#define repi(i, a, b, c)                                                       \
    for (long long i = (long long)(a); i < (long long)(b); i += c)
#define rep(...) _overload4(__VA_ARGS__, repi, _rep, __rep)(__VA_ARGS__)
#define _overload3_rev(_1, _2, _3, name, ...) name
#define _rep_rev(i, a) repi_rev(i, 0, a)
#define repi_rev(i, a, b)                                                      \
    for (long long i = (long long)(b)-1; i >= (long long)(a); --i)
#define rrep(...) _overload3_rev(__VA_ARGS__, repi_rev, _rep_rev)(__VA_ARGS__)

#define all(n) begin(n), end(n)
template <typename T, typename E> bool chmin(T &s, const E &t) {
    bool res = s > t;
    s = min<T>(s, t);
    return res;
}
template <typename T, typename E> bool chmax(T &s, const E &t) {
    bool res = s < t;
    s = max<T>(s, t);
    return res;
}
const vector<lint> dx = {1, 0, -1, 0, 1, 1, -1, -1};
const vector<lint> dy = {0, 1, 0, -1, 1, -1, 1, -1};
#define SUM(v) accumulate(all(v), 0LL)
#if __cplusplus >= 201703L
template <typename T, typename... Args>
auto make_vector(T x, int arg, Args... args) {
    if constexpr (sizeof...(args) == 0)
        return vector<T>(arg, x);
    else
        return vector(arg, make_vector<T>(x, args...));
}
#endif
#define extrep(v, ...) for (auto v : __MAKE_MAT__({__VA_ARGS__}))
#define bit(n, a) ((n >> a) & 1)
vector<vector<long long>> __MAKE_MAT__(vector<long long> v) {
    if (v.empty())
        return vector<vector<long long>>(1, vector<long long>());
    long long n = v.back();
    v.pop_back();
    vector<vector<long long>> ret;
    vector<vector<long long>> tmp = __MAKE_MAT__(v);
    for (auto e : tmp)
        for (long long i = 0; i < n; ++i) {
            ret.push_back(e);
            ret.back().push_back(i);
        }
    return ret;
}
using graph = vector<vector<int>>;
template <typename T> using graph_w = vector<vector<pair<int, T>>>;

#if __cplusplus >= 201703L
constexpr inline long long powll(long long a, long long b) {
    long long res = 1;
    while (b--)
        res *= a;
    return res;
}
#endif

template <typename T, typename E>
pair<T, E> &operator+=(pair<T, E> &s, const pair<T, E> &t) {
    s.first += t.first;
    s.second += t.second;
    return s;
}
template <typename T, typename E>
pair<T, E> &operator-=(pair<T, E> &s, const pair<T, E> &t) {
    s.first -= t.first;
    s.second -= t.second;
    return s;
}
template <typename T, typename E>
pair<T, E> operator+(const pair<T, E> &s, const pair<T, E> &t) {
    auto res = s;
    return res += t;
}
template <typename T, typename E>
pair<T, E> operator-(const pair<T, E> &s, const pair<T, E> &t) {
    auto res = s;
    return res -= t;
}
#define BEGIN_STACK_EXTEND(size)                                               \
    void *stack_extend_memory_ = malloc(size);                                 \
    void *stack_extend_origin_memory_;                                         \
    char *stack_extend_dummy_memory_ = (char *)alloca(                         \
        (1 + (int)(((long long)stack_extend_memory_) & 127)) * 16);            \
    *stack_extend_dummy_memory_ = 0;                                           \
    asm volatile("mov %%rsp, %%rbx\nmov %%rax, %%rsp"                          \
                 : "=b"(stack_extend_origin_memory_)                           \
                 : "a"((char *)stack_extend_memory_ + (size)-1024));
#define END_STACK_EXTEND                                                       \
    asm volatile("mov %%rax, %%rsp" ::"a"(stack_extend_origin_memory_));       \
    free(stack_extend_memory_);
int floor_pow(int n) { return n ? 31 - __builtin_clz(n) : 0; }
#line 2 "main.cpp"

int main() {
    lint n, m;
    cin >> n >> m;
    vec a(m);
    rep(i, m) cin >> a[i];
    lint ans = 0;
    rep(i, m - 1) {
        if (a[i + 1] - a[i] > 1) {
            lint sz = 1;
            auto f = [&](auto f, lint x) -> lint {
                if (x == 1)
                    return 1;
                if (x % 2)
                    return f(f, x / 2) * 2;
                else
                    return x;
            };
            ans += f(f, a[i + 1] - a[i] - 1);
        }
    }
    cout << ans << endl;
}