#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> arr(n);
  for (int i = 0; i < n; i++) cin >> arr[i];
  sort(arr.begin(), arr.end());
  int sign = 1;
  double ans = 0;
  for (int i = n - 1; i >= 0; i--) {
    ans += (sign * acos(-1) * arr[i] * arr[i]);
    sign *= -1;
  }
  cout << fixed << setprecision(10) << ans << endl;
}
