#include <bits/stdc++.h>
#define ll long long
#define ull unsigned long long
#define MP make_pair
#define PII pair<int, int>
#define PLL pair<ll, ll>
#define PLI pair<ll, int>
#define PIL pair<int, ll>
#define STR string
#define Fst first
#define Snd second
#define SHN 1e9
#define LSHN 1e18
using namespace std;
double n;
double r=3.1415926;
int main()
{
	cout <<fixed<<setprecision(6);
	cin >>n;
	cout <<n*2.0*r;
	return 0;
} 