#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7;
const long double PI = 3.141592653589793238462643383279502884197;
priority_queue<int, vector<int>, greater<int> > pq;
vector<int> v;
pair<int, int> vec[5000];
int num[5001];
int op[5001];
int tp[5001];
int main() {
  int n, q;
  scanf("%d %d", &n, &q);
  int sum = 0;
  for (int i = 0; i < q; i++) {
    scanf("%d %d", &vec[i].first, &vec[i].second);
    for (int j = vec[i].first; j <= vec[i].second; j++) num[j]++;
  }
  for (int i = 1; i <= n; i++) {
    if (num[i]) sum++;
    op[i] += op[i - 1] + (num[i] == 1);
    tp[i] += tp[i - 1] + (num[i] == 2);
  }
  int miv = 1e9;
  for (int i = 0; i < q; i++)
    for (int j = i + 1; j < q; j++) {
      v.clear();
      v.push_back(vec[i].first);
      v.push_back(vec[i].second);
      v.push_back(vec[j].first);
      v.push_back(vec[j].second);
      sort((v).begin(), (v).end());
      if ((vec[i].first <= vec[j].first && vec[i].second >= vec[j].first) ||
          (vec[j].first <= vec[i].first && vec[j].second >= vec[i].first)) {
        int t = 0;
        t += op[v[1] - 1] - op[v[0] - 1];
        t += op[v[3]] - op[v[2]];
        t += tp[v[2]] - tp[v[1] - 1];
        miv = min(miv, t);
      } else {
        int t = 0;
        t += op[v[1]] - op[v[0] - 1];
        t += op[v[3]] - op[v[2] - 1];
        miv = min(miv, t);
      }
    }
  printf("%d", sum - miv);
}
