#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vi = vector<int>;
using vb = vector<bool>;
using vll = vector<ll>;
using vd = vector<double>;
using vvi = vector<vi>;
using vvb = vector<vb>;
constexpr int MOD = 1000000007;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  double r;
  cin >> r;
  printf("%.10f\n", 2 * M_PI * r);
  return 0;
}
