#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<int, int>;
constexpr ld eps = 1e-12;
constexpr int inf = numeric_limits<int>::max() / 2;
constexpr int mod = 1e9 + 7;
template <typename T>
void printv(const vector<T> &v) {
  int sz = v.size();
  for (int i = 0; i < sz; i++) {
    cout << v[i] << " \n"[i == sz - 1];
  }
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ld r;
  cin >> r;
  cout << fixed << setprecision(10) << 2 * r * acos(-1.0) << endl;
}
