#include <bits/stdc++.h>
using namespace std;
const long long INF = 9e18;
const long long MOD = 1e9 + 7;
const long long mod = 998244353;
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
void solve() {
  long long n, m;
  cin >> n >> m;
  vector<long long> l(m), r(m);
  for (long long i = 0; i < m; i++) cin >> l[i] >> r[i];
  vector<long long> c(n + 1, 0);
  for (long long i = 0; i < m; i++) {
    for (long long j = l[i]; j < r[i] + 1; j++) c[j]++;
  }
  long long painted = 0;
  for (long long i = 1; i < n + 1; i++)
    if (c[i]) painted++;
  long long ans = 0;
  for (long long i = 0; i < m; i++) {
    for (long long j = l[i]; j < r[i] + 1; j++) {
      c[j]--;
      if (!c[j]) painted--;
    }
    vector<long long> ones(n + 1, 0);
    for (long long j = 1; j < n + 1; j++) {
      ones[j] = ones[j - 1];
      if (c[j] == 1) ones[j]++;
    }
    for (long long j = i + 1; j < m; j++) {
      long long x = ones[r[j]] - ones[l[j] - 1];
      ans = max(ans, painted - x);
    }
    for (long long j = l[i]; j < r[i] + 1; j++) {
      c[j]++;
      if (c[j] == 1) painted++;
    }
  }
  cout << ans << "\n";
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  solve();
}
