/* -*- coding: utf-8 -*-
 *
 * 1995.cc:  No.1995 CHIKA Road - yukicoder
 */

#include<cstdio>
#include<vector>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_M = 100000;
const int MAX_K = MAX_M * 2 + 2;

/* typedef */

typedef vector<int> vi;

/* global variables */

int as[MAX_M], bs[MAX_M], xs[MAX_K], dp[MAX_K];
vi nbrs[MAX_K];

/* subroutines */

inline void setmax(int &a, int b) { if (a < b) a = b; }

/* main */

int main() {
  int n, m;
  scanf("%d%d", &n, &m);

  int k = 0;
  xs[k++] = 1, xs[k++] = n;
  for (int i = 0; i < m; i++) {
    scanf("%d%d", as + i, bs + i);
    xs[k++] = as[i], xs[k++] = bs[i];
  }
  sort(xs, xs + k);
  k = unique(xs, xs + k) - xs;

  for (int i = 0; i < m; i++) {
    int ai = lower_bound(xs, xs + k, as[i]) - xs;
    int bi = lower_bound(xs, xs + k, bs[i]) - xs;
    nbrs[ai].push_back(bi);
  }

  for (int i = 0; i < k; i++) {
    if (i + 1 < n) setmax(dp[i + 1], dp[i]);
    for (auto j: nbrs[i]) setmax(dp[j], dp[i] + 1);
  }

  printf("%d\n", (n - 1) * 2 - dp[k - 1]);
  return 0;
}