#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  vector<double> r(N);
  for (int i = 0; i < N; i++) cin >> r[i];
  sort(r.begin(), r.end());
  reverse(r.begin(), r.end());
  r.push_back(0);
  double v = .0;
  for (int i = 0; i < r.size(); i += 2) {
    v += (3.141592653589793 * r[i] * r[i] -
          3.141592653589793 * r[i + 1] * r[i + 1]);
  }
  cout << fixed << setprecision(10) << v << endl;
  return 0;
}
