#include <bits/stdc++.h>
using namespace std;
void solve() {
  double r;
  cin >> r;
  cout << fixed;
  cout << setprecision(10) << (double)2 * 3.14159265358979323846 * r << endl;
}
int main() {
  long long int t = 1;
  while (t--) {
    solve();
    cout << endl;
  }
  return 0;
}
