#include <bits/stdc++.h>
using namespace std;
int main() {
  vector<vector<uint64_t>> arr(0);
  vector<uint64_t> temp_vec(2);
  vector<uint64_t> num_vec(0), red_vec(0), green_vec(0), white_vec(0);
  uint64_t num1, white;
  uint64_t num, result, result2;
  double length, mass;
  int64_t temp, temp2, temp3;
  int64_t i, j, k, l, count, n, x, y, o, size, a, b, c;
  cin >> x;
  result = 2 * x * M_PI;
  cout << result << endl;
  return 0;
}
