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