#include <iostream>
#include <math.h>
#include <vector>
#include <map>
#include <algorithm>
#include <deque>
#include <string>
#define _USE_MATH_DEFINES
#define pb push_back
#define f first
#define s second
#define mp make_pair
#define fors(i, a, b) for(int i = a; i < b; i++)
#define for(i, n) for(int i = 0; i < n; i++)
using namespace std;
typedef vector<int> vi;
typedef vector<string> vs;
typedef vector<pair<int, int> > vpi;
typedef vector<pair<string, int> > vpsi;
typedef map<string, int> msi;
int main()
{
    ios::sync_with_stdio(false);
    int n;
    cin >> n;
    cout << 2 * n * M_PI << endl;
}