#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<double> a(n + 1);
  for (int i = 1; i <= n; i++) cin >> a[i];
  a[0] = 0;
  sort(a.begin(), a.end());
  double ans = 0;
  int c = 1;
  for (int i = n; i >= 1; i--) {
    if (c % 2 == 1) ans += 3.1415926536 * (pow(a[i], 2) - pow(a[i - 1], 2));
    c++;
  }
  cout << ans;
  return 0;
}
