#include <bits/extc++.h>

int main() {
    using namespace std;
    unsigned long N, M;
    cin >> N >> M;
    vector<unsigned long> A(M);
    adjacent_difference(istream_iterator<unsigned long>{cin}, istream_iterator<unsigned long>{}, begin(A));
    A.erase(begin(A));
    
    unsigned long ans{};
    for(auto a : A){
        --a;
        unsigned long x{};
        while(a > 1 && a & 1){
            a /= 2;
            ++x;
        }
        ans += a << x;
    }
    cout << ans << endl;
    return 0;
}