#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const int N = 3e5 + 5;
const int K = (1 << 17) + 10;
const long long int INF = 2e18 + 500;
const int inf = 0x7fffffff;
const int SQRT = 500;
const int lgN = 20;
int l[N], r[N];
int main(int argc, char const *argv[]) {
  cin.sync_with_stdio(0);
  cin.tie(0);
  int n, q;
  cin >> n >> q;
  for (int i = 1; i <= q; i++) {
    cin >> l[i] >> r[i];
  }
  vector<int> f(n + 1, 0);
  vector<int> pf(n + 1, 0);
  int ans = 0;
  for (int i = 1; i <= q; i++) {
    vector<int> cnt(n + 2, 0);
    for (int j = 1; j <= q; j++) {
      if (i == j) continue;
      cnt[l[j]]++;
      cnt[r[j] + 1]--;
    }
    int sum = 0;
    for (int i = 1; i <= n; i++) {
      cnt[i] += cnt[i - 1];
      f[i] = (cnt[i] == 1);
      sum += cnt[i] > 0;
    }
    pf[0] = 0;
    for (int i = 1; i <= n; i++) {
      pf[i] = pf[i - 1] + f[i];
    }
    int v = inf;
    for (int j = 1; j <= q; j++) {
      if (j == i) continue;
      v = min(v, pf[r[j]] - pf[l[j] - 1]);
    }
    ans = max(ans, sum - v);
  }
  cout << ans << endl;
  return 0;
}
