#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  double r, up = 3.141592;
  cin >> r;
  cout << (up)*r + (up)*r << endl;
  return 0;
}
