#include <bits/stdc++.h>
using namespace std;
constexpr long long MOD = 1000000007;
constexpr long long INF = numeric_limits<long long>::max() / 2;
using Graph = vector<vector<long long>>;
signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  long long R;
  cin >> R;
  cout << fixed << setprecision(10);
  cout << 2 * R * 3.14159265 << endl;
}
