#include <bits/stdc++.h>
using namespace std;
using P = pair<long long, long long>;
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const long long MOD = 1000000007;
const int INF = 1 << 29;
int main() {
  double R;
  cin >> R;
  printf("%.6f", 2 * 3.14159265359 * R);
}
