#include<bits/stdc++.h>
using namespace std;

#define ll long long

int main()
{
	cin.tie(0)->sync_with_stdio(false);
	cin.exceptions(ios::badbit | ios::failbit);
	cout << fixed << setprecision(15);
	int th;
	cin >> th;
	auto cnt = [&](double r){
		long long res = 0;
		for(auto x = 1; x <= r; ++ x){
			res += sqrt(max(0.0, r * r - 1.0 * x * x));
		}
		return 4 * res;
	};
	double low = 0, high = 300000;
	for(auto rep = 300; rep > 0; -- rep){
		double mid = (low + high) / 2;
		if(cnt(mid) < th){
			low = mid;
		}
		else{
			high = mid;
		}
	}
	cout << (low + high) / 2 << "\n";
	return 0;
}