harmony

#include <bits/stdc++.h>

using namespace std;

void dbg_out() { cerr << endl; }
template<typename Head, typename... Tail>
void dbg_out(Head H, Tail... T) { cerr << ' ' << H; dbg_out(T...); }
#define dbg(...) cerr << "(" << #__VA_ARGS__ << "):", dbg_out(__VA_ARGS__)

#define rng_init mt19937 rng(chrono::steady_clock::now().time_since_epoch().count())
#define all(x) (x).begin(), (x).end()
#define sz(x) (int) (x).size()
// #define int long long

const int MXN = 2e5 + 5, INF = 1e9 + 5;
int par[MXN], compSz[MXN];

int get(int x) {
    if (par[x] == x) return x;
    return par[x] = get(par[x]);
}

bool unite(int x, int y) {
    x = get(x), y = get(y);
    if (x == y) return false;

    if (compSz[y] > compSz[x]) swap(x, y);
    compSz[x] += compSz[y];
    par[y] = x;

    return true;
}

void solve() {
    int N, M;
    cin >> N >> M;

    for (int i = 0; i <= N + 1; i++) {
        par[i] = i;
        compSz[i] = 1;
    }

    int ans = 0;

    vector<int> diff(N + 2, 0);
    vector<pair<int, int>> edges;

    for (int i = 0; i < M; i++) {
        int u, v;
        cin >> u >> v;
        if (u > v) swap(u, v);

        unite(u, v);

        edges.emplace_back(u, v);

        diff[u + 1]++;
        diff[v]--;
    }

    for (const auto &[u, v] : edges) {
        ans += unite(u, u + 1) + unite(v, v - 1);
    }

    for (int i = 1; i <= N; i++)
        diff[i] += diff[i - 1];

    for (int i = 1; i <= N; i++) {
//        dbg(i, diff[i]);
        if (diff[i] <= 0) continue;

        if (diff[i - 1] > 0) ans += unite(i - 1, i);
        if (diff[i + 1] > 0) ans += unite(i, i + 1);
    }

    cout << ans;
}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

#ifdef local
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif

    int TC = 1;
    // cin >> TC;
    while (TC--) solve();
}