c.cpp

#include <iostream>
#include <cassert>
#include <algorithm>
#include <numeric>
#include <vector>
#include <cstdlib>

typedef long long ll;

ll sum(ll n, ll k) {
    ll r = 0;
    while (n > 0) {
        ll curr = std::min(n, k);
        r += curr;
        n -= curr;
        n -= n / 10;
    }
    return r;
}

ll solve(ll n) {
    ll low = 0, high = n;
    while (high - low > 1) {
        ll mid = (low + high) / 2;
        if (2 * sum(n, mid) >= n) {
            high = mid;
        } else {
            low = mid;
        }
    }
    return high;
}

ll slow(ll n) {
    for (ll k = 1; k <= n; ++k) {
        if (2*sum(n,k) >= n) {
            return k;
        }
    }
    throw 1;
}

void test() {
    for (ll n = 1; n <= 100; ++n) {
        assert(slow(n) == solve(n));
    }
    std::cout << "ok!";
    std::exit(0);
}

int main() {
    //test();
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(0); std::cout.tie(0); std::cerr.tie(0);

    ll n;
    std::cin >> n;
    std::cout << solve(n);
    return 0;
}