#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using P = pair<int, int>;
const int INF = 1e9;
const double PI = 3.141592653589;
int main() {
  int r;
  cin >> r;
  cout << r * 2 * PI << endl;
}
