#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, a[100];
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
  }
  sort(a, a + n, greater<int>());
  int rr(0);
  for (int i = 0; i < n; ++i) {
    rr += a[i] * a[i] * (1 - i % 2 * 2);
  }
  double pi = asin(1) * 2;
  cout << fixed << setprecision(10) << (pi * rr) << endl;
  return 0;
}
