#include <bits/stdc++.h>
using namespace std;
int paintTheFence(vector<vector<int>>& grid, int n, int q) {
  vector<int> section[n + 1];
  vector<bool> covered(n + 1, false);
  vector<vector<int>> count(q, vector<int>(q, 0));
  for (int i = 0; i < q; i++) {
    int u = grid[i][0];
    int v = grid[i][1];
    for (int j = u; j <= v; j++) {
      if (section[j].size() < 3) section[j].push_back(i);
      covered[j] = true;
    }
  }
  int total = 0;
  for (int i = 1; i <= n; i++) {
    if (covered[i] == true) {
      total += 1;
    }
  }
  for (int i = 1; i <= n; i++) {
    if (section[i].size() == 1) {
      for (int j = 0; j < q; j++) {
        if (j != section[i][0]) {
          count[section[i][0]][j]++;
          count[j][section[i][0]]++;
        }
      }
    } else if (section[i].size() == 2) {
      count[section[i][0]][section[i][1]]++;
      count[section[i][1]][section[i][0]]++;
    }
  }
  int ans = INT_MAX;
  for (int i = 0; i < q; i++) {
    for (int j = i + 1; j < q; j++) {
      ans = min(ans, count[i][j]);
    }
  }
  return total - ans;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, q;
  cin >> n >> q;
  vector<vector<int>> grid(q, vector<int>(2, 0));
  for (int i = 0; i < q; i++) {
    for (int j = 0; j < 2; j++) {
      cin >> grid[i][j];
    }
  }
  cout << paintTheFence(grid, n, q) << endl;
}
