#include <bits/stdc++.h>
using namespace std;
using int32 = int_fast32_t;
using uint32 = uint_fast32_t;
using int64 = int_fast64_t;
using uint64 = uint_fast64_t;
using PII = pair<int32, int32>;
using PLL = pair<int64, int64>;
const double eps = 1e-10;
template <typename A, typename B>
inline void chmin(A &a, B b) {
  if (a > b) a = b;
}
template <typename A, typename B>
inline void chmax(A &a, B b) {
  if (a < b) a = b;
}
int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int64 n, q;
  cin >> n >> q;
  vector<int64> l(q), r(q);
  for (int64 i = 0; i < (q); i++) cin >> l[i] >> r[i];
  vector<int64> sum(n + 2, 0);
  for (int64 i = 0; i < (q); i++) {
    sum[l[i]]++;
    sum[r[i] + 1]--;
  }
  for (int64 i = 0; i < (n + 1); i++) sum[i + 1] += sum[i];
  int64 res = 0;
  for (int64 i = 0; i < (n + 2); i++) res += sum[i] > 0;
  int64 ans = 0;
  for (int64 i = 0; i < (q); i++) {
    vector<int64> sum2 = sum;
    int64 ret = res;
    for (int64 j = (l[i]); j < (r[i] + 1); j++) {
      sum2[j]--;
      if (sum2[j] == 0) ret--;
    }
    vector<int64> cnt1(n + 2, 0);
    for (int64 j = (1); j < (n + 2); j++)
      cnt1[j] += cnt1[j - 1] + (sum2[j] == 1);
    for (int64 j = (i + 1); j < (q); j++) {
      ans = max(ans, ret - (cnt1[r[j]] - cnt1[l[j] - 1]));
    }
  }
  cout << ans << endl;
}
