#include <bits/stdc++.h>
using namespace std;
double r[1000];
double p = atan(1.) * 4.;
int main() {
  double rez = 0;
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> r[i];
  sort(r, r + n);
  bool t = true;
  for (int i = n - 1; i >= 0; i--) {
    if (t)
      rez += r[i] * r[i] * p;
    else
      rez -= r[i] * r[i] * p;
    t = !t;
  }
  printf("%.6f", rez);
  cin >> n;
  return 0;
}
