#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
LL n;
LL test(LL k){
	// return number of turns it takes
	LL total = n;
	LL eaten = 0;
	LL rounds = 0;
	while(total > 0){
		LL a = min(total, k);
		total -= a;
		eaten += a;
		rounds++;
		total -= (total / 10);
	}
	if(eaten >= n-eaten){
		return 1;
	}
	return 0;
}
int main(){
	cin >> n;
	LL s = 0;
	LL e = 1000000000000000000LL; // yes
	while(s + 1 < e){
		LL m = (s+e)/2;
		if(test(m)){
			e = m;
		} else {
			s = m;
		}
	}
	cout << e << endl;
}

I2luY2x1ZGUgPGJpdHMvc3RkYysrLmg+CnVzaW5nIG5hbWVzcGFjZSBzdGQ7CnR5cGVkZWYgbG9uZyBsb25nIExMOwpMTCBuOwpMTCB0ZXN0KExMIGspewoJLy8gcmV0dXJuIG51bWJlciBvZiB0dXJucyBpdCB0YWtlcwoJTEwgdG90YWwgPSBuOwoJTEwgZWF0ZW4gPSAwOwoJTEwgcm91bmRzID0gMDsKCXdoaWxlKHRvdGFsID4gMCl7CgkJTEwgYSA9IG1pbih0b3RhbCwgayk7CgkJdG90YWwgLT0gYTsKCQllYXRlbiArPSBhOwoJCXJvdW5kcysrOwoJCXRvdGFsIC09ICh0b3RhbCAvIDEwKTsKCX0KCWlmKGVhdGVuID49IG4tZWF0ZW4pewoJCXJldHVybiAxOwoJfQoJcmV0dXJuIDA7Cn0KaW50IG1haW4oKXsKCWNpbiA+PiBuOwoJTEwgcyA9IDA7CglMTCBlID0gMTAwMDAwMDAwMDAwMDAwMDAwMExMOyAvLyB5ZXMKCXdoaWxlKHMgKyAxIDwgZSl7CgkJTEwgbSA9IChzK2UpLzI7CgkJaWYodGVzdChtKSl7CgkJCWUgPSBtOwoJCX0gZWxzZSB7CgkJCXMgPSBtOwoJCX0KCX0KCWNvdXQgPDwgZSA8PCBlbmRsOwp9

MTAwMDAwMDAwMDAwMDAwMDAwMA==

1000000000000000000