#include <bits/stdc++.h>
using namespace std;
class Graph {
 public:
  int V;
  vector<int> *adj;
  bool *visited;
  Graph(int V) {
    this->V = V;
    adj = new vector<int>[V];
    visited = new bool(V);
    memset(visited, 0, V * sizeof(bool));
  }
  void addEdge(int u, int v) {
    adj[u].push_back(v);
    adj[v].push_back(u);
  }
  void dfs(int u);
  void bfs(int u);
};
void Graph::dfs(int u) {
  if (visited[u]) return;
  visited[u] = true;
  cout << u << " ";
  for (auto a : adj[u]) {
    dfs(a);
  }
}
void Graph::bfs(int u) {
  queue<int> fila;
  fila.push(u);
  visited[u] = true;
  while (!fila.empty()) {
    int s = fila.front();
    cout << s << " ";
    fila.pop();
    for (auto a : adj[s]) {
      if (!visited[a]) {
        visited[a] = true;
        fila.push(a);
      }
    }
  }
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  double res;
  int value;
  cin >> value;
  res = (double)2 * value * (double)M_PI;
  cout << setprecision(10) << fixed << res << endl;
  return 0;
}
