#include<iostream>
using namespace std;
typedef long long ll;

bool judge(ll m, ll n){
    ll female = 0;
    ll male = 0;
    while(n>0){
        ll tmp = min(m, n);
        female += tmp;
        n -= tmp;
        tmp = n/10;
        male += tmp;
        n = n-tmp;
    }
    return female>=male;
}
int main(){

    ll n;  cin>>n;
    ll left = 1, right = n/2+1;
    while(left<right){
        ll mid = (right-left)/2+left;
        if(judge(mid, n)==true){
            right = mid;
        }else{
            left = mid+1;
        }
    }
    cout<<left<<endl;
    return 0;
}