#include <bits/stdc++.h>
using namespace std;
const int N = 5001;
short int dp[N][N][3];
short int n, q;
pair<short int, short int> ara[N];
short int Max(short int A, short int B) {
  if (A > B)
    return A;
  else
    return B;
}
short int call(short int pos, short int done, short int miss) {
  if (miss > 2) return -10000;
  if (pos == q + 1) {
    if (miss == 2)
      return 0;
    else
      return -10000;
  }
  if (dp[pos][done][miss] != -1) return dp[pos][done][miss];
  short int ret = call(pos + 1, done, miss + 1);
  if (ara[pos].first > done) {
    short int cc = ara[pos].second - ara[pos].first + 1;
    ret = Max(ret, cc + call(pos + 1, ara[pos].second, miss));
  } else if (ara[pos].second > done) {
    short int cc = ara[pos].second - done;
    ret =
        Max(ret, ara[pos].second - done + call(pos + 1, ara[pos].second, miss));
  } else {
    ret = max(ret, call(pos + 1, done, miss));
  }
  return dp[pos][done][miss] = ret;
}
bool cmp(pair<short int, short int> a, pair<short int, short int> b) {
  if (a.first == b.first) return a.second > b.second;
  return a.first < b.first;
}
int main() {
  memset(dp, -1, sizeof(dp));
  cin >> n >> q;
  for (int i = 1; i <= q; i++) {
    cin >> ara[i].first >> ara[i].second;
  }
  sort(ara + 1, ara + q + 1, cmp);
  cout << call(1, 0, 0) << "\n";
}
