#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
#define rep(i, n) for(int i=0;i<(n);++i)
#define rep1(i, n) for(int i=1;i<=(n);i++)
#define ll long long
using mint = modint998244353;
using P = pair<ll,ll>;
using lb = long double;
using T = tuple<ll, ll, ll>;
#ifdef LOCAL
#  include <debug_print.hpp>
#  define dbg(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#  define dbg(...) (static_cast<void>(0))
#endif

int main()
{
    ll n;
    cin >> n;
    int m;
    cin >> m;
    vector<ll> a(m);
    rep(i,m) cin >> a[i];
    ll ans = 0;
    auto f = [&](auto f, ll m) -> ll {
        ll ret = 0;
        if(m==1) return 1;
        if(m%2==0) return m;
        ret += f(f, m/2) * 2;
        return ret;
    };
    rep(i,m-1){
        ll val = a[i+1] - a[i] - 1;
        ans += f(f, val);
    }    
    cout << ans << endl;
    return 0;
}