func_code_string: #include <bits/stdc++.h>   using namespace std;   vector<long long> lucky;   void backtrack(long long x) {     if (x > 1e10) return;     lucky.push_back(x);     backtrack(x * 10 + 4);     backtrack(x * 10 + 7);   }   int main() {     long long ans = 0;     backtrack(0);     sort(lucky.begin(), lucky.end());     int a, b;     cin >> a >> b;     for (int i = 1; i < (int)lucky.size(); i++) {       int l, r, m;       l = a, r = b;       while (l <= r) {         m = (l + r) >> 1;         if (m <= lucky[i]) {           ans += lucky[i] * (m - l + 1);           a = m + 1;           i--;           break;         } else {           r = m - 1;         }       }     }     cout << ans <<   n ;     return 0;   }   