#include <bits/stdc++.h>
using namespace std;
int main() {
  long double pi = 3.1415926536;
  int n;
  cin >> n;
  int arr[n];
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
  }
  sort(arr, arr + n, greater<int>());
  int sum = 0;
  for (int i = 0; i < n; i++) {
    if (i % 2 == 0) {
      sum += pow(arr[i], 2);
    } else {
      sum -= pow(arr[i], 2);
    }
  }
  cout << fixed << setprecision(10) << pi * sum << endl;
}
