#include<bits/stdc++.h>
using namespace std;
typedef double db;
const db eps=1e-12;
typedef long long ll;

int s=0;

bool check(db mid)
{
	ll cnt=0;
	db d=mid*mid;
	for(int i=1;i<=mid;i++)
		cnt+=(sqrt(d-1ll*i*i)+(1e-12));
	return 4*cnt>s;
}

int main()
{
	scanf("%d",&s);
	db l=0,r=sqrt(s);
	while(l+eps<=r)
	{
		db mid=l+(r-l)*0.5;
		if(check(mid)) r=mid;
		else l=mid;
	}
	printf("%.20lf\n",r);
	return 0;
}