#include <bits/stdc++.h>
using namespace std;
void solve(int case_number) {
  double r;
  cin >> r;
  cout << fixed << setprecision(20);
  cout << r * 6.28318530717958623200 << endl;
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int cases = 1;
  for (int i = 0; i < cases; i++) solve(i + 1);
  return 0;
}
