func_code_string: #include <bits/stdc++.h>   using namespace std;   long long int n;   int main() {     cin >> n;     long long int summ = n + (n - 1);     long long int wei = 0;     while (summ != 0) {       summ /= 10;       wei++;     }     long long int stdl = 9;     for (int i = 1; i < wei; i++) {       stdl = stdl * 10 + 9;     }     summ = n + (n - 1);     if (summ < stdl) wei--, stdl /= 10;     long long int ans, p = stdl / 2;     ans = min(p, n - p);     long long int qian = 1;     for (int i = 1; i <= wei; i++) qian *= 10;     while (stdl + qian <= summ) {       stdl += qian;       p = stdl / 2;       ans += min(p, n - p);     }     if (n < 5) {       if (n == 2) ans = 1;       if (n == 3) ans = 3;       if (n == 4) ans = 6;     }     cout << ans;   }   