#include <bits/stdc++.h>
using namespace std;
bool compare(const pair<int, int>& i, const pair<int, int>& j) {
  return i.second < j.second;
}
void solve() {
  float r;
  cin >> r;
  cout << (2 * r * 3.1415) << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
