#include <bits/stdc++.h>
using namespace std;
long long int n;
int check(long long k) {
  long long patsum = 0, nn = n, pat;
  while (nn > 0) {
    nn = nn - k;
    pat = nn / 10;
    nn = nn - pat;
    patsum += pat;
    if (patsum > n / 2) {
      return 1;
    }
  }
  return 0;
}
long long bs() {
  long long left = 1, right = n, k;
  while (left < right) {
    k = (right + left) / 2;
    if (check(k)) {
      left = k + 1;
    } else
      right = k;
  }
  return left;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long t = 1, k, pat, patsum = 0, ori, flag = 0;
  while (t--) {
    cin >> n;
    long long ans = bs();
    cout << ans << endl;
  }
  return 0;
}
