#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <cmath>
#include <iomanip>
#include <stack>
#include <queue>
#include <numeric>
#include <map>
#include <unordered_map>
#include <set>
#include <fstream>
#include <chrono>
#include <random>
#include <bitset>
//#include <atcoder/all>
#include <sstream>
#define rep(i,n) for(int i=0;i<(n);i++)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) ((int)(x).size())
#define pb push_back
using ll = long long;
using namespace std;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
ll gcd(ll a, ll b) {return b?gcd(b,a%b):a;}
ll lcm(ll a, ll b) {return a/gcd(a,b)*b;}

int main(){
  int N,M; cin >> N >> M;
  set<int> st;
  st.insert(1);
  st.insert(N);
  map<int,vector<int>> G;
  map<int,vector<ll>> CO;
  rep(i,M){
    int a,b; cin >> a >> b;
    G[a].pb(b);
    CO[a].pb(2*b-2*a-1);
    st.insert(a);
    st.insert(b);
  }
  vector<int> vst;
  for(auto s:st){
    vst.pb(s);
  }
  rep(i,sz(vst)-1){
    int a = vst[i];
    int b = vst[i+1];
    G[a].pb(b);
    CO[a].pb(2*(b-a));
  }
  map<int,ll> D;
  set<int> used;
  priority_queue<pair<ll,int>,
                 vector<pair<ll,int>>,
                 greater<pair<ll,int>>> q;
  q.push({(ll)0,1});
  while(!q.empty()){
    ll d = q.top().first; int to = q.top().second; q.pop();
    if(used.count(to)) continue;
    used.insert(to); D[to] = d;
    rep(i,sz(G[to])){
      int nxt=G[to][i];
      if(D.count(nxt) && D[nxt] <= D[to] + CO[to][i]) continue;
      q.push({D[to] + CO[to][i], nxt});
    }
  }
  cout << D[N] << endl;
  return 0;
}