#include <bits/stdc++.h>
#pragma GCC optimize("O3")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
using namespace std;
const int MAX = 5005;
pair<int, int> pv[MAX];
int paintHeight[MAX];
int heightSum[MAX];
int twoPartSum[MAX];
int onePartSum[MAX];
int main() {
  int n, q;
  cin >> n >> q;
  for (int i = 0; i < q; i++) {
    int start, end;
    scanf("%d %d)", &start, &end);
    pv[i] = {start, end};
    paintHeight[start]++;
    paintHeight[end + 1]--;
  }
  int heightTmp = 0;
  for (int i = 0; i <= n; i++) {
    heightTmp += paintHeight[i];
    heightSum[i] = heightTmp;
  }
  int fullPaint = 0;
  for (int i = 1; i <= n; i++) {
    onePartSum[i] = onePartSum[i - 1] + (heightSum[i] == 1);
    twoPartSum[i] = twoPartSum[i - 1] + (heightSum[i] == 2);
    if (heightSum[i] > 0) fullPaint++;
  }
  sort(pv, pv + q);
  int minFence = 987654321;
  for (int i = 0; i < q; i++) {
    for (int j = i + 1; j < q; j++) {
      vector<int> v(4);
      v[0] = pv[i].first;
      v[1] = pv[i].second;
      v[2] = pv[j].first;
      v[3] = pv[j].second;
      sort(v.begin(), v.end());
      int oneFirst = onePartSum[v[1]] - onePartSum[v[0] - 1];
      int oneSecond = onePartSum[v[3]] - onePartSum[v[2] - 1];
      int two = 0;
      if ((pv[i].first <= pv[j].first && pv[j].first <= pv[i].second) ||
          (pv[i].first <= pv[j].second && pv[j].second <= pv[i].second)) {
        two = twoPartSum[v[2]] - twoPartSum[v[1] - 1];
      }
      minFence = min(minFence, oneFirst + oneSecond + two);
    }
  }
  printf("%d\n", fullPaint - minFence);
}
