#include <bits/stdc++.h>
#define sz(a) (int)a.size()
#define all(x) (x).begin(),(x).end()
#define pb push_back
#define mp make_pair
#define ll long long
#define f first
#define s second
#define y1 Emilia_Clarke
#define dracarys ios_base :: sync_with_stdio(0);cin.tie(0);cout.tie(0);
using namespace std;

const int inf = (int)1e9;
const int mod = (int)1e9 + 7;
const int N = 200200;


ll n;
bool can(ll k){
	ll sum = n;
	ll cur = 0;
	while(1){
		if(sum <= k){
			cur += sum;
			sum -= sum;
			break;
		}
		else{
			sum -= k;
			cur += k;
		}
	    if(sum/10 == 0){
	        cur += sum;
	        break;
	    }
	    else{
	    	sum -= sum/10;
	    }
	}
	ll D = n / 2;
	if(n % 2) D++;
	return cur >= D;
}
int main(){dracarys
	cin >> n;
	ll l = 1;
	ll r = n;
	ll res = -1;
	while(l <= r){
		ll mid = (l + r) >> 1;
		if(can(mid)){
			res = mid;
			r = mid - 1;
		}
		else
			l = mid + 1;
	}
	cout << res;
	return 0;
}