#include <bits/stdc++.h>
using namespace std;
const int N = 121;
const double Pi = 4 * atan(1);
int r[N];
int main() {
  ios ::sync_with_stdio(false);
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> r[i];
  sort(r + 1, r + n + 1);
  double ans = 0;
  for (int i = n; i >= 1; i -= 2)
    ans += (r[i] * r[i] - r[i - 1] * r[i - 1]) * Pi;
  cout << setprecision(6) << ans << '\n';
  return 0;
}
