#include <bits/stdc++.h>
using namespace std;
const int mMain = 5100;
int n, q;
int tedad[mMain];
int dp1[mMain], dp2[mMain];
int ll[mMain], rr[mMain];
int sum;
int ans;
int foo1(int l, int r) {
  if (r < l) return 0;
  l--;
  if (l < 0) return dp1[r];
  return dp1[r] - dp1[l];
}
int foo2(int l, int r) {
  if (r < l) return 0;
  l--;
  if (l < 0) return dp2[r];
  return dp2[r] - dp2[l];
}
struct Seg {
  int l, r;
};
Seg _2ta(int l1, int r1, int l2, int r2) { return {max(l1, l2), min(r1, r2)}; }
void getMax(int first, int second) {
  int l1 = ll[first];
  int l2 = ll[second];
  int r1 = rr[first];
  int r2 = rr[second];
  if (l1 > l2) {
    swap(l1, l2);
    swap(r1, r2);
  }
  if (l2 > r1)
    ans = max(ans, sum - foo1(l1, r1) - foo1(l2, r2));
  else {
    Seg gz = _2ta(l1, r1, l2, r2);
    ans = max(ans, sum - foo1(min(l1, l2), gz.l - 1) - foo2(gz.l, gz.r) -
                       foo1(gz.r + 1, max(r1, r2)));
  }
}
int main() {
  iostream::sync_with_stdio(0);
  cin >> n >> q;
  for (int i = 0; i < q; i++) {
    int l, r;
    cin >> l >> r;
    l--, r--;
    rr[i] = r;
    ll[i] = l;
    for (int j = l; j <= r; j++) tedad[j]++;
  }
  for (int i = 0; i < n; i++) {
    if (tedad[i] <= 1) dp1[i] = 1;
    if (i != 0) dp1[i] += dp1[i - 1];
    if (tedad[i] <= 2) dp2[i] = 1;
    if (i != 0) dp2[i] += dp2[i - 1];
    if (tedad[i] >= 1) sum++;
  }
  for (int i = 0; i < q - 1; i++)
    for (int j = i + 1; j < q; j++) getMax(i, j);
  cout << ans;
}
