#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3fffffff;
const long long MOD = 1000000007;
const long long MODD = 998244353;
template <typename T>
bool chmax(T &m, const T q) {
  if (m < q) {
    m = q;
    return true;
  } else
    return false;
}
template <typename T>
bool chmin(T &m, const T q) {
  if (m > q) {
    m = q;
    return true;
  } else
    return false;
}
int main() {
  double long R;
  cin >> R;
  cout << 2 * 3.14 * R << "\n";
}
