#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  double l = 0, p;
  p = 3.141592653589793;
  int a[n + 5];
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  sort(a + 1, a + n + 1);
  for (int i = n; i >= 1; i--) {
    l += (pow(-1, i + (n % 2)) * (a[i] * a[i]));
  }
  cout << fixed << setprecision(9) << l * p;
  return 0;
}
