#include <bits/stdc++.h>
using namespace std;
const long long nax = 1e5 + 5;
vector<long long> adj[nax];
const long long mod = 1e9 + 7;
const long long inf = 1e18 + 7;
signed main() {
  long long n;
  cin >> n;
  std::cout << std::setprecision(15) << 2 * 3.14159265358979323846 * n << "\n";
  return 0;
}
