#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int INF = 1e9 + 5;
void print_vector2(vector<vector<ll> > v) {
  for (ll i = 0; i < v.size(); i++) {
    for (ll j = 0; j < v[i].size(); j++) {
      cout << v[i][j] << " ";
    }
    cout << "\n";
  }
}
void print_vector1(vector<ll> v) {
  for (ll i = 0; i < v.size(); i++) {
    cout << v[i] << " ";
  }
  cout << "\n";
}
ll gcd(ll a, ll b) {
  if (a == 0) return b;
  if (b == 0) return a;
  if (a == b) return a;
  if (a > b) return gcd(a - b, b);
  return gcd(a, b - a);
}
int main() {
  double r;
  cin >> r;
  cout << r * r * 22 / 7;
}
