#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <atcoder/modint>

using namespace std;
using i32 = int32_t;
using u32 = uint32_t;
using i64 = int64_t;
using u64 = uint64_t;
#define rep(i,n) for(int i=0; i<(int)(n); i++)


const i64 INF = 1001001001001001001;
using modint = atcoder::static_modint<1000000007>;

template<class T> using nega_queue = priority_queue<T, vector<T>, greater<T>>;


struct Edge { int to; int dist; };


int main(){
	int N; cin >> N; N--;
	int M; cin >> M;
	vector<int> idxs;
	vector<int> A(M);
	vector<int> B(M);
	rep(i,M){
		cin >> A[i]; A[i]--;
		cin >> B[i]; B[i]--;
	}
	rep(i,M){
		idxs.push_back(A[i]);
		idxs.push_back(B[i]);
	}
	idxs.push_back(0);
	idxs.push_back(N);

	sort(idxs.begin(), idxs.end());
	idxs.erase(unique(idxs.begin(), idxs.end()), idxs.end());
	auto getidx = [&](int x) -> int { return lower_bound(idxs.begin(), idxs.end(), x) - idxs.begin(); };

	int n = idxs.size();
	vector<vector<Edge>> adj(n);
	rep(i,M){
		adj[getidx(A[i])].push_back({ getidx(B[i]), 2*B[i] - 2*A[i] - 1 });
	}
	rep(i,n-1){
		adj[i].push_back({ i+1, 2*idxs[i+1] - 2*idxs[i] });
	}

	vector<i64> dist(n, INF);
	dist[0] = 0;
	rep(i,n) for(auto edge : adj[i]) dist[edge.to] = min(dist[edge.to], dist[i] + edge.dist);
	
	cout << dist.back() << '\n';
    return 0;
}


struct ios_do_not_sync{
    ios_do_not_sync(){
        std::ios::sync_with_stdio(false);
        std::cin.tie(nullptr);
    }
} ios_do_not_sync_instance;