#include <bits/stdc++.h>
using namespace std;
using ll = int64_t;
void solve();
void getInput();
int64_t W, H;
double pi = 3.1415926535;
void getInput() {}
void solve() {
  int64_t R;
  cin >> R;
  cout << 2 * R * pi << endl;
}
signed main() {
  getInput();
  solve();
  return 0;
}
