#include <bits/stdc++.h>
using namespace std;
template <typename T1, typename T2>
inline void chmin(T1 &a, T2 b) {
  if (a > b) a = b;
}
template <typename T1, typename T2>
inline void chmax(T1 &a, T2 b) {
  if (a < b) a = b;
}
const int MOD = 1000000007;
void solve() {
  double r;
  cin >> r;
  cout << r * 2 * 3.14 << endl;
}
int main() {
  solve();
  return 0;
}
