#include <bits/stdc++.h>
using namespace std;
long long int power(long long int x, long long int y, long long int p) {
  long long int res = 1;
  x = x % p;
  while (y > 0) {
    if (y & 1) res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
long long int fact[2 * 100005];
long long int func() {
  fact[0] = fact[1] = 1;
  for (long long int i = 2; i < 200001; i++) {
    fact[i] = (fact[i - 1] * i) % 1000000007;
  }
  return 0;
}
int L[5005];
int R[5005];
int s[5005], t[5005];
int main() {
  int n, q;
  cin >> n >> q;
  for (int i = 0; i < q; i++) {
    cin >> L[i] >> R[i];
    for (int j = L[i]; j <= R[i]; j++) {
      s[j]++;
    }
  }
  int ans = 0;
  for (int i = 0; i < q; i++) {
    for (int j = L[i]; j <= R[i]; j++) {
      s[j]--;
    }
    int a = 0;
    for (int j = 1; j <= n; j++) {
      t[j] = (s[j] == 1) ? t[j - 1] + 1 : t[j - 1];
      if (s[j] >= 1) a++;
    }
    int tmp = INT_MAX;
    for (int j = 0; j < q; j++) {
      if (j == i) continue;
      tmp = min(tmp, t[R[j]] - t[L[j] - 1]);
    }
    ans = max(ans, a - tmp);
    for (int j = L[i]; j <= R[i]; j++) {
      s[j]++;
    }
  }
  cout << ans;
}
