#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  double a[n];
  for (int i = 0; i < n; i++) cin >> a[i];
  sort(a, a + n);
  double area = 0;
  for (int i = n - 1; i > 0; i = i - 2)
    area += ((3.141592653589793238462 * a[i] * a[i]) -
             (3.141592653589793238462 * a[i - 1] * a[i - 1]));
  if (n % 2 == 1) area += (3.141592653589793238462 * a[0] * a[0]);
  printf("%.15f", area);
  return 0;
}
