#include <iostream>
#include <string>
#include <math.h>
#include <vector>
#include <algorithm>
#include <map>
#include <set>
#include <stack>
#include <queue>

using namespace std;
typedef long long ll;
#define rep(i,n) for(ll i=0;i<(ll)n;i++)
#define rep1(i,n) for(ll i=1;i<(ll)n;i++)
#define repf(i,a,b) for(ll i=(ll)a;i<(ll)b;i++)
#define repr(i,a,b) for(ll i=(ll)a-1;i>=(ll)b;i--)
#define Fi first
#define Se second
#define PI 3.14159265359
typedef pair<ll, ll> pll;
const ll MAX = 1000000000000000000;
const char END = '\n';


int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);

	double R, Ans;
	cin >> R;
	
	Ans = R * 2 * PI;

	cout << Ans << endl;

	return 0;
}