#include <bits/stdc++.h>   using namespace std;   bool KT(long long k, long long n) {     long long take = 0, cur = n;     while (cur > 0) {       long long temp = min(k, cur);       take += temp;       cur -= temp;       cur -= cur / 10;     }     if (take * 2 >= n) return true;     return false;   }   int main() {     long long n;     cin >> n;     long long left = 1, right = n, res = 0;     while (left <= right) {       long long mid = (left + right) / 2;       if (KT(mid, n)) {         res = mid;         right = mid - 1;       } else {         left = mid + 1;       }     }     cout << res;     return 0;   }   