#include <bits/stdc++.h>
using namespace std;
int index(vector<int>& v1, int s, int e) {
  int p = s;
  int p_e = v1[e];
  for (int i = s; i < e; i++) {
    if (v1[i] <= p_e) {
      swap(v1[i], v1[p]);
      p++;
    }
  }
  swap(v1[e], v1[p]);
  return p;
}
void quick_sort(vector<int>& v1, int s, int e) {
  if (s <= e) {
    int p_index = index(v1, s, e);
    quick_sort(v1, s, p_index - 1);
    quick_sort(v1, p_index + 1, e);
  }
}
int main() {
  int n;
  cin >> n;
  vector<int> v(n + 1);
  v[0] = 0;
  for (int i = 1; i <= n; i++) {
    int r;
    cin >> r;
    v[i] = r;
  }
  quick_sort(v, 1, n);
  int sum = 0;
  for (int i = (n % 2 == 0) ? 2 : 1; i <= n; i += 2) {
    sum += ((v[i] * v[i]) - (v[i - 1] * v[i - 1]));
  }
  cout << fixed << setprecision(10) << sum * 2 * asin(1.0) << endl;
  v.clear();
}
