#include <cstddef>
#include <numbers>
#include <iostream>

bool check_ratio(const int ratio)
{
  bool result = true;
  if (ratio < 1) {
    result = false;
  }
  else if (100 < ratio) {
    result = false;
  }
  return result;
}

int main()
{
  int ratio;
  std::cin >> ratio >> std::endl;
  if (!check_ratio(ratio)) {
    std::cout << "invarid input as ratio. (1 <= ratio <= 100)" << std::endl;
  }
  else {
    std::cout << 2 * ratio * std::numbers::pi << std::endl;
  } 
}