#include <bits/stdc++.h>
using namespace std;
int mp0[5050], mp1[5050], mp2[5050];
int x[5050], y[5050];
int gett[5050];
int main() {
  int n, q;
  scanf("%d%d", &n, &q);
  for (int i = 1; i <= q; i++) {
    scanf("%d%d", &x[i], &y[i]);
    for (int j = x[i]; j <= y[i]; j++) gett[j]++;
  }
  int sum0 = 0;
  for (int k = 1; k <= n; k++) {
    if (gett[k] == 0) sum0++;
    if (gett[k] == 1)
      mp1[k] = mp1[k - 1] + 1;
    else
      mp1[k] = mp1[k - 1];
    if (gett[k] == 2)
      mp2[k] = mp2[k - 1] + 1;
    else
      mp2[k] = mp2[k - 1];
  }
  int maxn = 0;
  for (int i = 1; i < q; i++) {
    for (int j = i + 1; j <= q; j++) {
      int x1 = x[i], x2 = x[j], y1 = y[i], y2 = y[j];
      if (x1 > x2) swap(x1, x2), swap(y1, y2);
      if (x2 <= y1) {
        if (y2 <= y1) {
          int sum_x = 0;
          sum_x += mp1[x2 - 1] - mp1[x1 - 1] + mp1[y1] - mp1[y2];
          sum_x += mp2[y2] - mp2[x2 - 1];
          maxn = max(maxn, n - sum_x - sum0);
        }
        if (y2 > y1) {
          int sum_x = 0;
          sum_x += mp1[x2 - 1] - mp1[x1 - 1] + mp1[y2] - mp1[y1];
          sum_x += mp2[y1] - mp2[x2 - 1];
          maxn = max(maxn, n - sum_x - sum0);
        }
      } else {
        int sum_x = 0;
        sum_x += mp1[y1] - mp1[x1 - 1] + mp1[y2] - mp1[x2 - 1];
        maxn = max(maxn, n - sum_x - sum0);
      }
    }
  }
  cout << maxn << endl;
}
