#include <stdio.h>
#include <cstdio>
#include <iostream>
#include <iomanip>
#include <queue>
#include <set>
#include <vector>
#include <string>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <complex>

using ll = long long int;
using namespace std;

int main(){
  double pi = 6.28318530717958623200;
  ll R;
  cin >> R;

  cout << setprecision(15) << R * pi << endl;

  return 0;
}
