#include <bits/stdc++.h>
using namespace std;
const long long MAXN = 5005;
const long long INF = 0x3f3f3f3f;
const long long LINF = 0x3f3f3f3f3f3f3f3f;
const long long MOD = 1e9 + 7;
const long long OVER_FLOW = 0xffffffff;
long long n, q;
vector<int> v[MAXN];
int a[MAXN], b[MAXN][MAXN];
int main() {
  ios::sync_with_stdio(false);
  cin >> n >> q;
  for (long long i = 1; i <= q; i++) {
    long long l, r;
    cin >> l >> r;
    for (long long j = l; j <= r; j++) v[j].push_back(i);
  }
  long long sm = 0;
  for (long long i = 1; i <= n; i++) {
    long long m = v[i].size();
    if (m > 0) sm++;
    if (m == 1)
      a[v[i][0]]++;
    else if (m == 2)
      b[v[i][0]][v[i][1]]++;
  }
  long long res = 0;
  for (long long i = 1; i <= q; i++) {
    for (long long j = i + 1; j <= q; j++) {
      res = max(res, sm - a[i] - a[j] - b[i][j]);
    }
  }
  cout << res << endl;
  return 0;
}
