#include <iostream>
#include <vector>
#include <algorithm>
#include <unordered_set>
#include <unordered_map>
#include <sstream>
#include <set>
#include <string>
#include <math.h>
#include <map>
#include <climits>
#include <iomanip>


#define forn(i, n) for (int i = 0; i < int(n); i++)

#define ll long long
#define _USE_MATH_DEFINES

using namespace std;

void A();
void B();
void C();
void D();
void E();
void F();

int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	cout.tie(NULL);

	A();


}

void A() {
	int n;
	cin >> n;

	double cir = n * 2 * 3.14159265358979323846;
	cout << setprecision(8) << cir << endl;
}
 
void B() {
	int T;
	cin >> T;
	for (int t = 1; t <= T; t++) {

		int N;
		cin >> N;


		int result = 0;

		cout << "Case #" << t << ": ";

		cout << result << endl;

	}
}

void C() {
	int T;
	cin >> T;
	for (int t = 1; t <= T; t++) {

		int N;
		cin >> N;


		int result = 0;

		cout << "Case #" << t << ": ";

		cout << result << endl;

	}
}

void D() {
	int T;
	cin >> T;
	for (int t = 1; t <= T; t++) {

		int N;
		cin >> N;


		int result = 0;

		cout << "Case #" << t << ": ";

		cout << result << endl;

	}
}

void E() {
	int T;
	cin >> T;
	for (int t = 1; t <= T; t++) {

		int N;
		cin >> N;


		int result = 0;

		cout << "Case #" << t << ": ";

		cout << result << endl;

	}
}

void F() {
	int T;
	cin >> T;
	for (int t = 1; t <= T; t++) {

		int N;
		cin >> N;


		int result = 0;

		cout << "Case #" << t << ": ";

		cout << result << endl;

	}
}

