#include <bits/stdc++.h>
using ll = long long;
using namespace std;
ll GCD(ll a, ll b) { return a ? GCD(b % a, a) : b; }
int main() {
  double r;
  cin >> r;
  cout << fixed << setprecision(10) << (double)2.0 * r * 3.141592 << endl;
}
