#include <bits/stdc++.h>
using namespace std;
const int INF = 1000000007;
const long long INFLL = (long long)INF * (long long)INF;
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int r;
  double circum;
  cin >> r;
  circum = 2 * 3.1415926535 * r;
  cout << circum;
  return 0;
}
