#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vll = vector<ll>;
using vvll = vector<vector<ll>>;
using P = pair<ll, ll>;
const ll mod = 1e9 + 7;
int main() {
  cin.tie(0);
  cout.tie(0);
  ios::sync_with_stdio(false);
  double pi = 3.14159;
  double r;
  cin >> r;
  cout << 2 * pi * r << endl;
  return 0;
}
