#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  double r[1100];
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> r[i];
  }
  sort(r, r + n);
  double area = 0;
  for (int i = n - 1, j = 1; i >= 0; i--, j++) {
    if (j % 2 == 1) {
      area += (acos(-1) * r[i] * r[i]);
    } else {
      area -= (acos(-1) * r[i] * r[i]);
    }
  }
  cout << setprecision(15) << fixed << area << endl;
}
