#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;
    rep(i,m-1){
        ll val = a[i+1] - a[i] - 1;
        if(val%2==0){
            ans += val;
        }
        else{
            if(val==1){
                ans++;
            }
            else if(val%4==1){
                ans += val-1;
            }
            else{
                ans += (val-1)/2+1;
            }
            // ans += (val+1)/2;
        }
        dbg(ans);
        dbg(val);
    }    
    cout << ans << endl;
    return 0;
}