#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  double c = 0;
  cin >> n;
  int a[n];
  for (int i = 0; i < n; ++i) cin >> a[i];
  sort(a, a + n);
  if (n % 2) {
    c = a[0] * a[0];
    for (int i = 2; i < n; i += 2) {
      c = c + a[i] * a[i] - a[i - 1] * a[i - 1];
    }
    cout << fixed << setprecision(10) << 3.14159265358979323846 * c;
  } else {
    for (int i = n - 1; i >= 1; i -= 2)
      c = c + a[i] * a[i] - a[i - 1] * a[i - 1];
    cout << fixed << setprecision(10) << 3.14159265358979323846 * c;
  }
  return 0;
}
