func_code_string: #include <bits/stdc++.h>   using namespace std;   uint64_t l, r, s;   set<uint64_t> S;   void f(uint64_t x) {     S.insert(x);     if (x < r) f(x * 10 + 4), f(x * 10 + 7);   }   int main() {     cin >> l >> r;     for (f(0); l <= r;) {       auto it = S.lower_bound(l);       s += (min(r, *it) - l + 1) * *it;       l = *it + 1;     }     cout << s <<   n ;   }   