#include <bits/stdc++.h>
using namespace std;
int arr[1000000], arr2[1000000], ans[1000000];
map<int, int> mop, mopp;
bool hna[1000000];
unsigned long long ncr(unsigned long long n, unsigned long long r) {
  if (r == 0) return 1;
  if (n == 0) return 0;
  return (n * ncr(n - 1, r - 1) / r);
}
int main() {
  int n;
  scanf("%d", &n);
  ;
  long long ans = 0;
  for (int i = 0; i < n; i++) {
    scanf("%d", &arr[i]);
    ;
  }
  sort(arr, arr + n);
  for (int i = n - 1; i >= 0; i -= 2) {
    if (i == 0) {
      ans += arr[i] * arr[i];
    } else {
      ans += (arr[i] * arr[i]) - (arr[i - 1] * arr[i - 1]);
    }
  }
  cout << setprecision(10) << fixed << ans * 3.1415926535897933846153846153846
       << endl;
  return 0;
}
