#include <bits/stdc++.h>
#define ALL(x) (x).begin(), (x).end()
#define LB(v, x) (int)(lower_bound(ALL(v), x) - (v).begin())
#define UQ(v) sort(ALL(v)), (v).erase(unique(ALL(v)), (v).end())
#define IO ios::sync_with_stdio(false), cin.tie(nullptr);
#define chmax(a, b) (a) = (a) < (b) ? (b) : (a)
#define chmin(a, b) (a) = (a) < (b) ? (a) : (b)
using namespace std;
using ll = long long;
const int INF = 1e9 + 10;
const ll INFL = 4e18;

int main() {
    ll N, M;
    cin >> N >> M;
    vector<ll> S(M);
    for (int i = 0; i < M; i++) cin >> S[i], S[i]--;

    vector<ll> to(N);
    vector<pair<int, int>> ans;
    iota(ALL(to), 0);
    for (int i = M - 1; i >= 0; i--) {
        ans.push_back(minmax(to[S[i]], to[S[i] + 1]));
        swap(to[S[i]], to[S[i] + 1]);
    }
    UQ(ans);

    cout << ssize(ans) << endl;
}