#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[n];
  for (int i = 0; i < n; i++) cin >> a[i];
  sort(a, a + n);
  int x = 0, y = 0;
  for (int i = n - 1; i >= 0; i -= 2) x += a[i] * a[i];
  for (int i = n - 2; i >= 0; i -= 2) y += a[i] * a[i];
  double pi = 3.1415926535;
  cout << std::setprecision(10) << pi * (x - y);
  return 0;
}
