#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)
using namespace std;
using ll = long long;
using P = pair<int, int>;

int main()
{
  double N;
  cin >> N;
  const double PI = acos(-1);
  cout << 2 * PI * N << endl;
}