#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

const double PI = 3.14159265358979323846;

int main() {
  int r;
  cin >> r;
  double ans = (double)2.0 * (double)r * PI;
  cout << ans << endl;
  return 0;
}