#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;
}
