#include <bits/stdc++.h>
using namespace std;
struct node {
  int x;
  int y;
} b[5005];
int vis[5005][5] = {0};
int cnt[5005] = {0};
bool cmp(node n, node m) {
  if (n.x == m.x) {
    return n.y < m.y;
  }
  return n.x < m.x;
}
int judge(int x, int y, int sum) {
  if (b[x].y < b[y].x) {
    sum -= (vis[b[x].y][1] - vis[b[x].x - 1][1]);
    sum -= (vis[b[y].y][1] - vis[b[y].x - 1][1]);
  } else {
    int xx = max(b[x].x, b[y].x);
    int yy = min(b[x].y, b[y].y);
    sum -= (vis[b[x].y][1] - vis[b[x].x - 1][1]);
    sum -= (vis[b[y].y][1] - vis[b[y].x - 1][1]);
    sum -= (vis[yy][2] - vis[xx - 1][2]);
  }
  return sum;
}
int main() {
  int n, m;
  cin >> n >> m;
  for (int i = 1; i <= m; i++) {
    cin >> b[i].x >> b[i].y;
    for (int j = b[i].x; j <= b[i].y; j++) {
      cnt[j]++;
    }
  }
  sort(b + 1, b + 1 + m, cmp);
  int sum = 0;
  for (int i = 1; i <= n; i++) {
    vis[i][1] += vis[i - 1][1];
    vis[i][2] += vis[i - 1][2];
    if (cnt[i] == 1) {
      vis[i][1]++;
    } else if (cnt[i] == 2) {
      vis[i][2]++;
    }
    if (cnt[i]) sum++;
  }
  int ans = 0;
  for (int i = 1; i <= m; i++) {
    for (int j = i + 1; j <= m; j++) {
      ans = max(ans, judge(i, j, sum));
    }
  }
  cout << ans << endl;
  return 0;
}
