#include <bits/stdc++.h>
using namespace std;
long long MOD = 1e9 + 7, MAX = 1e18, k = 1e9;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ;
  long long r;
  double ans;
  cin >> r;
  ans = (22 * r * r * 1.0) / 7;
  cout << fixed << setprecision(10) << ans;
  return 0;
}
