#include <bits/stdc++.h>
using namespace std;
bool descendingSort(int a, int b) { return a > b; }
int main() {
  int N;
  cin >> N;
  double PI = 3.141592653589793;
  int cir[N];
  memset(cir, 0, sizeof(cir));
  for (int i = 0; i < N; i++) cin >> cir[i];
  sort(cir, cir + N, descendingSort);
  double ans = 0.0;
  bool isRed = true;
  for (int i = 0; i < N; i++) {
    if (isRed) {
      ans += double(cir[i] * cir[i]) * PI;
      isRed = false;
    } else {
      ans -= double(cir[i] * cir[i]) * PI;
      isRed = true;
    }
  }
  cout << fixed << showpoint << setprecision(6) << ans << endl;
  return 0;
}
