#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
template <class T>
inline int chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline int chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
const int INF = 1 << 29;
const ll LINF = 1LL << 58;
const int mod = 1000000007;
void solve() {
  double r;
  cin >> r;
  cout << fixed << setprecision(10) << 3.141592 * 2 * r << endl;
  return;
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
  return 0;
}
