#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n;
  cin >> n;
  long long arr[n];
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
  }
  sort(arr, arr + n);
  double r = 0;
  for (int i = n - 1; i >= 0; i -= 2) {
    if (i != 0) {
      r += (arr[i] * arr[i] - arr[i - 1] * arr[i - 1]);
    } else {
      r += (arr[i] * arr[i]);
    }
  }
  double ans = 3.1415926536;
  ans *= r;
  cout << fixed << setprecision(11) << ans;
}
