#include <bits/stdc++.h>
using namespace std;
const int INF = 1 << 30;
const long long INFL = 1LL << 60;
const int MOD = 1000000007;
const int MAX = 100000;
const int N = 100010;
int main() {
  cout << fixed << setprecision(15);
  const long double pi = 3.14159265;
  long double r;
  cin >> r;
  cout << 2 * r * pi << endl;
}
