#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[1000];
  for (int i = 0; i < n; i++) cin >> a[i];
  sort(a, a + n);
  reverse(a, a + n);
  int s = 0;
  for (int i = 0; i < n; i++)
    if (i % 2 == 0)
      s += a[i] * a[i];
    else
      s -= a[i] * a[i];
  cout << 3.14159264 * s;
}
