#include <bits/stdc++.h>
using namespace std;
long long multiple(long long a, long long b, long long c) {
  if (b == 0) return 0;
  long long ret = multiple(a, b >> 1, c);
  ret = (ret + ret) % c;
  if (b & 1) ret = (ret + a) % c;
  return ret;
}
vector<pair<long long, long long> > v;
long long a[5005], h[5005], one[5005], two[5005];
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long long n, q, final = 0, maxx, l1, l2, r2, r1, x, y, i, j;
  cin >> n >> q;
  for (i = 0; i < q; i++) {
    cin >> x >> y;
    v.push_back({x, y});
    for (j = x; j <= y; j++) {
      h[j]++;
    }
  }
  one[0] = 0;
  two[0] = 0;
  final = 0;
  for (i = 1; i <= n; i++) {
    one[i] = one[i - 1] + (h[i] == 1);
    two[i] = two[i - 1] + (h[i] == 2);
    if (h[i] >= 1) {
      final++;
    }
  }
  maxx = 0;
  for (i = 0; i < q; i++) {
    for (j = i + 1; j < q; j++) {
      l1 = v[i].first;
      r1 = v[i].second;
      l2 = v[j].first;
      r2 = v[j].second;
      if (l2 < l1) {
        swap(l1, l2);
        swap(r1, r2);
      }
      if (l2 > r1) {
        maxx = max(maxx,
                   final - (one[r1] - one[l1 - 1]) - (one[r2] - one[l2 - 1]));
        continue;
      }
      maxx = max(maxx, final - (two[min(r1, r2)] - two[l2 - 1]) -
                           (one[l2 - 1] - one[l1 - 1]) -
                           (one[max(r1, r2)] - one[min(r1, r2)]));
    }
  }
  cout << maxx;
}
