#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
const ll INF = 1LL << 60;
const int INF32 = 1 << 29;
const ll MOD = 1000000007;
using Graph = vector<vector<int>>;
using pint = pair<int, int>;
int main() {
  int R;
  cin >> R;
  cout << R * 2 * 3.141592 << endl;
}
