491C

#include <bits/stdc++.h>

using namespace std;

bool check(long long k, long long n) {
    long long sum = 0;
    long long cur = n;
    while (cur > 0) {
        long long o = min(cur, k);
        sum += o;
        cur -= o;
        cur -= cur / 10;
    }
    return sum * 2 >= n;
}

int main() {
    long long n;
    cin >> n;

    long long l = 1, r = n;
    long long ans = r;
    while (l <= r) {
        long long k = (l + r) / 2;
        if (check(k, n)) {
            ans = k;
            r = k - 1;
        }
        else
            l = k + 1;
    }

    cout << ans << endl;
}