#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
using i64 = long long;
using u64 = unsigned long long;
#define rep(i,n) for(int i=0; i<(int)(n); i++)
const i64 INF = 1001001001001001001;

void testcase(){
    i64 N, M; cin >> N >> M;
    vector<i64> A(M); rep(i,M) cin >> A[i];
    i64 ans = 0;
    rep(i,M-1){
        i64 a = A[i+1] - A[i] - 1;
        i64 t = 1;
        while(a != 1 && a%2 == 1){
            a /= 2; t *= 2;
        }
        ans += a * t;
    }
    cout << ans << endl;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    testcase();
    return 0;
}