#include <bits/stdc++.h>
using namespace std;
using pii = pair<int, int>;
using ll = long long;
const ll MOD = 1000000007;
const ll INFL = (1LL << 60);
const int INFI = (1 << 30);
int main() {
  double R;
  cin >> R;
  cout << 3.1415926535 * R << endl;
}
