#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int arr[n];
  for (int i = 0; i < int(n); i++) cin >> arr[i];
  sort(arr, arr + n);
  int area = 0;
  if (n % 2 == 0) {
    for (int i = 1; i < int(n); i++) {
      area += ((arr[i] * arr[i]) - (arr[i - 1] * arr[i - 1]));
      i++;
    }
  } else {
    area = arr[0] * arr[0];
    for (int i = 2; i < int(n); i++) {
      area += ((arr[i] * arr[i]) - (arr[i - 1] * arr[i - 1]));
      i++;
    }
  }
  cout << (double)area * 3.1415926536;
  cout << endl;
}
