#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#define _GLIBCXX_DEBUG
using namespace std;
using ll=long long;
using ld=long double;
ll ILL=2167167167167167167;
const int INF=2100000000;
const ll mod=998244353;
#define rep(i,a) for (ll i=0;i<a;i++)
#define all(p) p.begin(),p.end()
template<class T> using _pq = priority_queue<T, vector<T>, greater<T>>;
template<class T> ll LB(vector<T> &v,T a){return lower_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> ll UB(vector<T> &v,T a){return upper_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> bool chmin(T &a,const T &b){if(a>b){a=b;return 1;}else return 0;}
template<class T> bool chmax(T &a,const T &b){if(a<b){a=b;return 1;}else return 0;}
template<class T> void So(vector<T> &v) {sort(v.begin(),v.end());}
template<class T> void Sore(vector<T> &v) {sort(v.begin(),v.end(),[](T x,T y){return x>y;});}
void yneos(bool a){if(a) cout<<"Yes\n"; else cout<<"No\n";}
template<class T> void vec_out(vector<T> &p){for(int i=0;i<(int)(p.size());i++){if(i) cout<<" ";cout<<p[i];}cout<<"\n";}

using len_type=ll;
struct edge{
    int to;
    len_type length;
};

len_type dijkstra_movement(edge e,int from,len_type t){
    return t+e.length;
}


vector<len_type> dijkstra(vector<vector<edge>> &G,int start,len_type inf){
    int M=G.size();
    assert(M>start&&start>=0);
    vector<len_type> seen(M,inf);
    seen[start]=0;
    priority_queue<pair<len_type,int>,vector<pair<len_type,int>>,greater<pair<len_type,int>>> pq;
    pq.push({0,start});
    while(!pq.empty()){
        len_type time;
        int ver;
        tie(time,ver)=pq.top();
        pq.pop();
        if(seen[ver]<time) continue;
        for(edge x:G[ver]){
            len_type T=dijkstra_movement(x,ver,time);
            if(T<seen[x.to]){
                pq.push({T,x.to});
                seen[x.to]=T;
            }
        }
    }
    return seen;
}


void solve();
// 飄々とエモーション
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	
	int t=1;
	//cin>>t;
	rep(i,t) solve();
}

void solve(){
	int N,M;
	cin>>N>>M;
	set<int> s;
	vector<int> A(M),B(M);
	s.insert(1),s.insert(N);
	int ind=0,f=-1;
	map<int,int> m;
	vector<vector<edge>> G;
	rep(i,M){
		cin>>A[i]>>B[i];
		s.insert(A[i]);
		s.insert(B[i]);
	}
	for(auto x:s){
		G.push_back({});
		m[x]=ind;
		if(f!=-1){
			G[ind].push_back({ind-1,2*(x-f)});
			G[ind-1].push_back({ind,2*(x-f)});
		}
		ind++;
		f=x;
	}
	rep(i,M){
		int a=m[A[i]],b=m[B[i]],len=(B[i]-A[i])*2-1;
		G[a].push_back({b,len});
		G[b].push_back({a,len});
	}
	auto ans=dijkstra(G,0,INF);
	cout<<ans[ind-1]<<"\n";
}