#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  double r;
  cin >> r;
  double ans = (double)2 * ((double)(3.14159)) * r;
  cout << ans;
  return 0;
}
