#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
#include <fstream>
#include <string>
#include <math.h>
#include <cstdlib>
#include <cstdint>
#include <istream>
#include <sstream>
#include <cctype>
#define fore(i,x,y) for(ll i=(x);i<(y);i++)
#define disp(a) cout << a << endl;
typedef long long ll;
using namespace std;

bool comp(const int& a, const int& b) {
	return a > b;
}

int main()
{
	double r;
	cin >> r;
	cout << 2*r * 3.14159265358979323846 << endl;

}