#include<bits/stdc++.h>
#include <unordered_map>
#define int long long
#define ll long long 
#define rep(i, n) for(ll i = 0; i < (n); i++)
#define rep2(i, n) for(ll i = 0; n; i++)
#define rep3(i,s,n) for(ll i = (s); i < (n); i++)
#define rep4(i,s,n) for(ll i = (s); n; i++)
#define repe(v, V) for(auto& v: V)
#define all(x) (x).begin(),(x).end()
#define v vector
#define mod(a,b) (((a) % (b) + (b)) % (b)) /* 負の数に対応した剰余 */
#define pi 3.14159265358979
#define eps 1e-13
#define inf 9223372036854775807
#define MAX_LL 9223372036854775807
#define umap unordered_map
using namespace std;
const int dx[4] = { 1, 0, -1, 0 };
const int dy[4] = { 0, 1, 0, -1 };
template<typename T> using min_priority_queue = priority_queue<T, vector<T>, greater<T>>;
int to_base10(string Nx, int base)  /* N進数から10進数に変換 */ { int N10 = 0; rep(i, Nx.length()) N10 += stoll(Nx.substr(Nx.length() - 1 - i, 1)) * pow(base, i); return N10; }
string to_baseN(int N10, int base, int min_digits = 1)  /* 10進数からN進数に変換 */ { string r; while (N10 != 0) { r = to_string(N10 % base) + r; N10 /= base; } while (r.length() < min_digits) r = "0" + r; return r; }
v<pair<int, int>> prime_factorize(int N) /* 素因数分解 .first: 素数, .second: 指数 */ { v<pair<int, int>> res; rep4(a, 2, a * a <= N) { if (N % a != 0) continue; int ex = 0; while (N % a == 0) { ++ex; N /= a; } res.push_back({a, ex}); } if (N != 1) res.push_back({N, 1}); return res; }
v<int> divisors(int N)/*約数列挙*/ {v<int> r;rep4(i, 1, i * i <= N) {if (N % i == 0) {r.push_back(i);if(i != N / i) r.push_back(N / i);}}return r;}
int modinv(int a, int m)/*逆元*/ {	int b = m, u = 1, v = 0;while (b) {	long long t = a / b;a -= t * b; swap(a, b);	u -= t * v; swap(u, v);	}u %= m;if (u < 0) u += m;	return u;}
int sigma(int a, int b)/*a,a+1,...,bの総和*/ {return b * (b + 1) / 2 - a * (a - 1) / 2;}
v<v<int>> Pascal(int N, int mod = MAX_LL)/*パスカルの三角形(nCk=v[n][k]) O(N^2)*/ { v<v<int>> com(N + 1,v<int>(N + 1)); com[0][0] = 1; for (int i = 1; i < N + 1; ++i) { com[i][0] = 1; for (int j = 1; j < N + 1; j++) { com[i][j] = (com[i - 1][j - 1] + com[i - 1][j]) % mod; } }return com; }
int Pow(int n, int p, int mod = MAX_LL)/* 指数関数のLL版（繰り返し2乗法） modあり */ {int ans = 1;int now = n;for (; p > 0; p >>= 1){if ((p & 1) == 1){ans *= now;ans %= mod;}now *= now;now %= mod;}return ans;}
void printVec(const vector<int>& vec)/* vectorの中身をprint */ {cout << "";for (auto it = vec.begin(); it != vec.end(); ++it) {cout << *it << " ";	}	cout << endl;}
void makeset(v<int>& vec)/* vectorをソートし、重複要素を削除*/ { sort(vec.begin(), vec.end());	vec.erase(unique(vec.begin(), vec.end()), vec.end());}

int N, M;

signed main()
{
	cout << setprecision(15);

	cin >> N >> M;
	v<int> A(M), B(M), NodeList(0);
	rep(i, M) {
		cin >> A[i] >> B[i];
		NodeList.push_back(A[i]);
		NodeList.push_back(B[i]);
	}
	NodeList.push_back(1);
	NodeList.push_back(N);
	makeset(NodeList);

	int N2 = NodeList.size();

	v<v<int>> G(N2), W(N2);

	rep(i, N2 - 1) {
		G[i].push_back(i + 1);
		W[i].push_back((NodeList[i + 1] - NodeList[i]) * 2);
	}

	rep(i, M) {
		int a = lower_bound(all(NodeList), A[i]) - NodeList.begin();
		int b = lower_bound(all(NodeList), B[i]) - NodeList.begin();
		G[a].push_back(b);
		W[a].push_back(2 * (B[i] - A[i]) - 1);
	}

	queue<int> que; //stackにすればDFS
	vector<int> dist(G.size(), MAX_LL);

	que.push(0);
	dist[0] = 0;

	while (!que.empty()) {
		int i = que.front();
		que.pop();

		for (int j = 0; j < G[i].size(); j++) {
			if (dist[G[i][j]] > dist[i] + W[i][j]) {
				dist[G[i][j]] = dist[i] + W[i][j];
				que.push(G[i][j]);
			}
		}
	}

	cout << dist[N2 - 1];

}