#include <bits/stdc++.h>
using namespace std;
using lint = long long;
using p = pair<int, int>;
template <class T>
bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
int main() {
  double N;
  cin >> N;
  double pi = 3.14159265;
  double ans = pi * N * 2;
  cout << ans << endl;
  return 0;
}
