#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, sum = 0, a[100 + 10];
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  sort(a, a + n);
  for (int i = n - 1; i >= 0; i -= 2) sum += a[i] * a[i];
  for (int i = n - 2; i >= 0; i -= 2) sum -= a[i] * a[i];
  cout << sum * 3.1415926536;
  return 0;
}
