#include <bits/stdc++.h>
using namespace std;
const int fx[4][2] = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
const int fxx[8][2] = {{0, 1}, {0, -1}, {1, 0},  {-1, 0},
                       {1, 1}, {1, -1}, {-1, 1}, {-1, -1}};
const int rogi[4][2] = {{1, 1}, {1, -1}, {-1, 1}, {-1, -1}};
double n;
void dane() {
  cin >> n;
  cout << setprecision(9) << (2 * M_PI * n) << endl;
}
int main() {
  ios_base::sync_with_stdio(0);
  dane();
  return 0;
}
