#include <bits/stdc++.h>
#define inf 2000000000
using namespace std;
using Pair = pair<int, int>;


int main(){
    int n, m;   cin >> n >> m;
    vector<pair<int, int> > edges(m);
    for(auto&[x, y] : edges){
        cin >> x >> y;
        --x, --y;
    }

    vector<int> V = {0, n - 1};
    for(auto&[x, y] : edges){
        V.push_back(x);
        V.push_back(y);
    }
    sort(V.begin(), V.end());
    V.erase(unique(V.begin(), V.end()), V.end());
    int N = (int)V.size();
    unordered_map<int, int> mp = {};
    for(int i = 0; i < N; ++i){
        mp[V[i]] = i;
    }

    vector<vector<Pair> > graph(N, vector<Pair>({}));
    for(int i = 0; i < N; ++i){
        graph[i].push_back({i + 1, 2 * (V[i + 1] - V[i])});
    }
    for(auto&[x, y] : edges){
        graph[mp[x]].push_back({mp[y], 2 * y - 2 * x - 1});
    }

    vector<int> dp(N, inf);
    priority_queue<Pair, vector<Pair>, greater<Pair> > pq;
    pq.push({0, 0});
    while(!pq.empty()){
        auto[c, v] = pq.top();  pq.pop();
        if(dp[v] <= c){
            continue;
        }
        dp[v] = c;
        if(v == N - 1){
            break;
        }
        for(auto&[nv, nd] : graph[v]){
            pq.push({c + nd, nv});
        }
    }
    cout << dp[N - 1] << endl;

    return 0;
}