#include <bits/stdc++.h>
#pragma GCC optimize(2)
using namespace std;
const int mod = 1e9 + 7;
const int MAX = 2e5 + 7;
const double pi = 6.28318530717958623200;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  double r;
  cin >> r;
  cout << fixed << setprecision(20) << r * pi;
}
