#include <bits/stdc++.h>
using ll = long long;
using namespace std;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
void printV(std::vector<long long> const &input) {
  for (long long i = 0; i < input.size(); i++) std::cout << input.at(i) << ' ';
}
signed main(void) {
  long long R;
  cin >> R;
  long double temp = 2 * R * M_PI;
  cout.precision(32);
  cout << temp << endl;
  return 0;
}
