func_code_string: #include <bits/stdc++.h>   using namespace std;   const long long N = 2e5 + 5, inf = 1e9 + 7;   long long n, k, i, j;   long long C[N], f[3][N];   pair<long long, long long> A[N];   inline long long util(long long a, long long b, long long c) {     return f[c][a] - f[c][b];   }   signed main() {     ios_base::sync_with_stdio(false);     cin.tie(0);     long long i, j, a, b, ans = 0, m, temp, c, d, e, ff, cur = 0;     cin >> n >> k;     for (i = 0; i < k; i++) {       cin >> A[i].first >> A[i].second;       C[A[i].first]++;       C[A[i].second + 1]--;     }     sort(A, A + k);     for (i = 1; i < n + 2; i++) C[i] += C[i - 1];     for (i = 1; i < n + 1; i++)       if (C[i]) cur++;     for (i = 1; i < n + 1; i++)       for (j = 1; j < 3; j++)         if (C[i] == j) f[j][i]++;     for (i = 1; i < n + 1; i++)       for (j = 1; j < 3; j++) f[j][i] += f[j][i - 1];     temp = 0;     for (i = 0; i < k; i++)       for (j = i + 1; j < k; j++) {         if (A[i].second < A[j].first) {           temp = cur - util(A[i].second, A[i].first - 1, 1) -                  util(A[j].second, A[j].first - 1, 1);         } else if (A[i].second <= A[j].second) {           a = A[i].first - 1;           b = A[j].first - 1;           c = A[j].first - 1;           d = A[i].second;           e = A[i].second;           ff = A[j].second;           temp = cur - util(b, a, 1) - util(d, c, 2) - util(ff, e, 1);         } else {           a = A[i].first - 1;           b = A[j].first - 1;           c = A[j].first - 1;           d = A[j].second;           e = A[j].second;           ff = A[i].second;           temp = cur - util(b, a, 1) - util(d, c, 2) - util(ff, e, 1);         }         ans = max(ans, temp);       }     cout << ans;   }   