#include <bits/stdc++.h>
using namespace std;
using ll = 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;
}
const ll INF = 1LL << 60;
int main() {
  int r;
  cin >> r;
  double ans = 2 * M_PI * r;
  printf("%.10f\n", ans);
  return 0;
}
