func_code_string: #include <bits/stdc++.h>   using namespace std;   bool sortbysec(const pair<long long, long long> &a,                  const pair<long long, long long> &b) {     return (a.second < b.second);   }   inline long long mx(long long a, long long b) {     if (a > b)       return a;     else       return b;   }   long long gcd(long long a, long long b) {     if (b == 0)       return a;     else       return gcd(b, a % b);   }   long long power(long long x, long long y, long long p) {     long long res = 1;     x = x % p;     while (y > 0) {       if (y & 1) res = (res * x) % p;       y = y >> 1;       x = (x * x) % p;     }     return res;   }   long long solve(long long x, long long n) {     long long r;     if (x <= n)       r = (x - 1) / 2;     else       r = (n - (x - n) + 1) / 2;     if (r > 0)       return r;     else       return 0;   }   int main() {     std::ios_base::sync_with_stdio(false);     cin.tie(NULL);     long long n;     cin >> n;     long long i;     for (i = 9; i <= n + n - 1; i = i * 10 + 9)       ;     long long k = i / 10;     long long ans = 0;     for (long long i = 0; i <= 9; i++) {       if (i * (k + 1) + k <= n + n - 1)         ans += solve(i * (k + 1) + k, n);       else         break;     }     cout << ans <<   n ;     return 0;   }   