func_code_string: #include <bits/stdc++.h>   using namespace std;   const long long M = 1e10;   set<long long> s;   void bt(long long x) {     if (x >= M) return;     s.insert(x);     bt(x * 10 + 4);     bt(x * 10 + 7);   }   int main() {     bt(0);     int l, r;     cin >> l >> r;     long long c = 0;     for (set<long long>::iterator it = s.begin(); it != s.end(); it++) {       if (*it >= l) {         if (r <= *it) {           c += (r - l + 1) * *it;           break;         } else           c += (*it - l + 1) * *it;         l = *it + 1;       }     }     cout << c << endl;     return 0;   }   