#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n;
  cin >> n;
  int arr[n];
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
  }
  sort(arr, arr + n);
  double sum = 0;
  if (n % 2 == 0) {
    for (int i = 0; i < n; i += 2) {
      sum += (arr[i + 1] * arr[i + 1] - arr[i] * arr[i]);
    }
    cout << sum * (3.141592653);
  } else {
    for (int i = 1; i < n; i += 2) {
      sum += (arr[i + 1] * arr[i + 1] - arr[i] * arr[i]);
    }
    cout << (sum + arr[0] * arr[0]) * (3.141592653);
  }
}
int main() {
  solve();
  return 0;
}
