#include <bits/stdc++.h>
using namespace std;
const long long mod = 1000000007;
const long long LINF = 1LL << 60;
const int INF = 1 << 30;
int main() {
  double r;
  cin >> r;
  cout << 2.00000000 * r * 3.141592 << endl;
  return 0;
}
