#include <bits/stdc++.h>
using namespace std;
void solve();
long long int ans;
int main(int argc, char const *argv[]) {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  solve();
}
void solve() {
  long double R;
  cin >> R;
  cout << 2 * 3.14159 * R << '\n';
}
