#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using PII = pair<ll, ll>;
template <typename T>
void chmin(T &a, const T &b) {
  a = min(a, b);
}
template <typename T>
void chmax(T &a, const T &b) {
  a = max(a, b);
}
struct FastIO {
  FastIO() {
    cin.tie(0);
    ios::sync_with_stdio(0);
  }
} fastiofastio;
constexpr ll INF = 1LL << 60;
int main() {
  ll r;
  cin >> r;
  cout << fixed << setprecision(9) << 2 * r * acos(-1) << endl;
  return 0;
}
