#include <bits/stdc++.h>
using namespace std;
const int MAX = 1e5 + 55;
int x[MAX];
int main() {
  int n;
  cin >> n;
  double sum = 0;
  int a[n + 1];
  for (int i = 0; i < n; i++) cin >> a[i];
  sort(a, a + n, greater<int>());
  for (int i = 0; i < n; i++) {
    if (i % 2 == 0)
      sum += a[i] * a[i];
    else
      sum -= a[i] * a[i];
  }
  cout << setprecision(11) << fixed << sum * 3.1415926536;
  return 0;
}
