#include<iostream>
#include<vector>
#include<string>
#include<algorithm>
#include <cmath>
#include <math.h>

using namespace std;
const long long INF = (long long) (1e18)+1;

int main() {
  double n;
  cin >> n;
  double ans = 2*n*M_PI;
  cout << ans << endl;
  return 0;
}