#include <bits/stdc++.h>
using namespace std;
const long long inf = 1e9 + 7;
const long long mod = 998244353;
signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  double r;
  cin >> r;
  cout << fixed << setprecision(10) << r * M_PI * 2.00 << '\n';
}
