#include <bits/stdc++.h>
using namespace std;

#define REP(i, n) for (int i = 0; i < (n); i++)

int main() {
    long long N, M;
    cin >> N >> M;
    vector<long long> A(M);
    REP(i, M) cin >> A[i];
    vector<long long> B(M - 1);
    REP(i, M - 1) B[i] = A[i + 1] - A[i] - 1;
    map<long long, long long> mp;
    auto rec = [&](auto f, long long x) -> long long {
        if (mp.count(x) > 0) return mp[x];
        if (x == 0) return 0;
        if (x == 1) return 1;
        if (x == 2) return 2;
        if (x % 2 == 0) return x;
        return mp[x] = 2 * f(f, x / 2);
    };
    long long ans = 0;
    REP(i, M - 1) ans += rec(rec, B[i]);
    cout << ans << '\n';
    return 0;
}