func_code_string: #include <bits/stdc++.h>   using namespace std;   const long long MAX = 1e9;   vector<long long> lucky_nums;   void generate_lucky(long long n) {     if (n > MAX) return;     long long n1 = 10 * n + 4;     lucky_nums.push_back(n1);     generate_lucky(n1);     long long n2 = 10 * n + 7;     lucky_nums.push_back(n2);     generate_lucky(n2);   }   void solve() {     generate_lucky(0);     sort(lucky_nums.begin(), lucky_nums.end());     long long l, r;     cin >> l >> r;     int i =         lower_bound(lucky_nums.begin(), lucky_nums.end(), l) - lucky_nums.begin();     long long prev = l, ans = 0;     while (lucky_nums[i] < r) {       ans += (lucky_nums[i] - prev + 1) * lucky_nums[i];       prev = lucky_nums[i] + 1;       i++;     }     ans += (r - prev + 1) * lucky_nums[i];     cout << ans << endl;   }   int main() {     solve();     return 0;   }   