#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
int dx4[4] = {1, 0, -1, 0}, dy4[4] = {0, 1, 0, -1};
int main() {
  std::cout << std::fixed;
  std::cout << std::setprecision(6);
  double d;
  cin >> d;
  cout << 2.0 * d * M_PI << endl;
  return 0;
}
