func_code_string: #include <bits/stdc++.h>   using namespace std;   const long long N = 5000;   long long cumulative[N + 10], arr[N + 10][3];   signed main() {     ios ::sync_with_stdio(false);     cin.tie(0);     cout.tie(0);     long long n, q;     cin >> n >> q;     vector<pair<long long, long long> > pi;     for (long long i = 0; i < q; i++) {       long long a, b;       cin >> a >> b;       cumulative[a] += 1;       cumulative[b + 1] -= 1;       pi.push_back({a, b});     }     for (long long i = 1; i <= n; i++) {       cumulative[i] += cumulative[i - 1];     }     for (long long i = 1; i <= n; i++) {       for (long long j = 0; j < 3; j++) {         if (cumulative[i] == j) {           arr[i][j] += (arr[i - 1][j] + 1);         } else {           arr[i][j] += arr[i - 1][j];         }       }     }     long long sz = pi.size();     sort(pi.begin(), pi.end());     long long ans = 0;     for (long long i = 0; i < sz - 1; i++) {       long long x = pi[i].first;       long long y = pi[i].second;       for (long long j = i + 1; j < sz; j++) {         long long two = 0;         long long a = pi[j].first;         long long b = pi[j].second;         if (a <= y) {           long long ax = max(x, a);           long long by = min(y, b);           two = (arr[by][2] - arr[ax - 1][2]);         }         long long zero = (arr[n][0] - arr[0][0]);         long long one = (arr[y][1] - arr[x - 1][1]) + (arr[b][1] - arr[a - 1][1]);         long long total = one + two + zero;         long long res = n - total;         ans = max(ans, res);       }     }     cout << ans << endl;     return 0;   }   