#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9;
const int MOD = 1e9 + 7;
const long long LINF = 1e18;
int main() {
  double r;
  cin >> r;
  cout << setprecision(11) << (double)2 * r * 3.14159265358979323846 << endl;
}
