#include <iostream>
#include <string>
#include <map>
#include <vector>
#include <iomanip>
#include <math.h>
#include <mutex>
#include <algorithm>

//#include "test.h"

//#include "SandBox/StopRangeAccess/StopRangeAccess.h"

//#include "Data.h"


using namespace std;

typedef unsigned long ulong;
typedef unsigned long long ulolong;
typedef long long lolong;


int main()
{
	int r;
	cin >> r;

	cout << r * 3.141592;
}