#include <bits/stdc++.h>
using namespace std;
double a[100000], s, w;
int n;
int main() {
  double p = 3.1415926536;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  sort(a, a + n);
  s = pow(a[0], 2) * p;
  for (int i = n - 1; i > 0; i -= 2)
    w += (pow(a[i], 2) * p) - (pow(a[i - 1], 2) * p);
  if (n % 2 == 0) {
    cout << fixed << setprecision(10) << w;
    return 0;
  }
  cout << fixed << setprecision(10) << w + s;
  return 0;
}
