#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  long long n;
  cin >> n;
  long long arr[n];
  for (long long i = 0; i < n; i++) cin >> arr[i];
  double pi = 3.1415926535;
  sort(arr, arr + n);
  double ans = 0;
  if (n % 2 != 0) {
    for (long long i = 0; i < n; i += 2) {
      if (i == 0)
        ans += (pi * pow(arr[i], 2));
      else
        ans += (pi * (pow(arr[i], 2) - pow(arr[i - 1], 2)));
    }
  } else {
    for (long long i = 1; i < n; i += 2)
      ans += (pi * (pow(arr[i], 2) - pow(arr[i - 1], 2)));
  }
  cout << fixed << setprecision(10) << ans << endl;
}
