#include <bits/stdc++.h>
using namespace std;
template <typename T>
void print(T t) {
  for (const auto& e : t) cout << e << " ";
  cout << endl;
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long t, n, k, r;
  cin >> r;
  cout << (double)2 * 3.14159265358 * r << "\n";
  return 0;
}
