func_code_string: #include <bits/stdc++.h>   using namespace std;   const int N = 1e6 + 7;   const int M = 22;   const long long mod = 1e9 + 7;   mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());   int main() {     ios_base::sync_with_stdio(0);     cin.tie(0);     cout.tie(0);     long long n;     cin >> n;     long long s = 0;     while (s * 10 + 9 <= n + n - 1) {       s = s * 10 + 9;     }     if (s == n + n - 1) {       cout << 1;     } else {       long long len = 1;       while (len * 1ll * 10 <= 1ll * n + n - 1ll) {         len *= 10;       }       long long i = s;       long long ans = 0;       for (; i <= n + n - 1; i += len) {         if (i <= n + 1) {           ans += (i - 1) / 2;         } else {           long long x = i - n;           x = n - x + 1;           ans += x / 2;         }       }       cout << ans;     }   }   