#include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;
const int INF = 1e9;
const long double M_PI = 3.141592653589793;
int main() {
  ll n;

  cin >> n;

  long double ans = 2 * n * M_PI;
  cout << setprecision(100)<<ans << endl;
  return 0;
}
