#include <bits/stdc++.h>
using namespace std;

void fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
}

int main() {
    fast_io();
    int n, m;
    cin >> n >> m;
    vector<int> a(m), b(m), val;
    for (int i = 0; i < m; i++) {
        cin >> a[i] >> b[i];
        val.push_back(a[i]);
        val.push_back(b[i]);
    }
    val.push_back(1);
    val.push_back(n);
    sort(val.begin(), val.end());
    val.erase(unique(val.begin(), val.end()), val.end());
    int K = val.size();
    vector<vector<pair<int, int>>> g(K);
    for (int i = 0; i < K - 1; i++) {
        g[i].push_back({i + 1, 2 * (val[i + 1] - val[i])});
    }
    for (int i = 0; i < m; i++) {
        int u = lower_bound(val.begin(), val.end(), a[i]) - val.begin();
        int v = lower_bound(val.begin(), val.end(), b[i]) - val.begin();
        g[u].push_back({v, 2 * b[i] - 2 * a[i] - 1});
    }
    vector<long long> dp(K, 1e18);
    dp[0] = 0;
    for (int i = 0; i < K; i++) {
        for (auto [j, w] : g[i]) {
            dp[j] = min(dp[j], dp[i] + w);
        }
    }
    cout << dp[K - 1] << "\n";
}