#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
void solve() {
  float r;
  cin >> r;
  float pi = 3.14159;
  float ans = (float)2 * pi * r;
  cout << fixed << setprecision(4) << ans << "\n";
}
int main() {
  solve();
  return 0;
}
