#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using Graph = vector<vector<int>>;
const ll LINF = 1e18;
const int INF = 1e9;
const ll MOD = 1000000007;
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
vector<bool> seen;
void dfs(const Graph& G, int v) {
  seen[v] = true;
  for (auto next_v : G[v]) {
    if (seen[next_v]) continue;
    dfs(G, next_v);
  }
}
int main() {
  double r;
  cin >> r;
  double pi = 3.1415926535;
  cout << 2 * r * pi << endl;
  return 0;
}
