#include <bits/stdc++.h>
using namespace std;
double pi = 3.1415926536;
int main() {
  int n;
  cin >> n;
  int r[n];
  for (int i = 0; i < n; i++) cin >> r[i];
  double area = 0;
  sort(r, r + n);
  for (int i = n - 1; i >= 0; i -= 2) {
    if (i > 0)
      area += (r[i] * r[i] - r[i - 1] * r[i - 1]);
    else
      area += r[0] * r[0];
  }
  area *= pi;
  printf("%0.10lf", area);
  return 0;
}
