#include <bits/stdc++.h>
using namespace std;
double PI = 3.1415926536;
int n, j;
double r[100 + 9], prevArea, ans, area;
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) cin >> r[i];
  sort(r, r + n);
  area = 0;
  if (n % 2 == 0) j = 1;
  for (int i = j; i < n; i += 2) {
    if (i)
      area += ((r[i] * r[i]) - (r[i - 1] * r[i - 1]));
    else
      area += (r[i] * r[i]);
  }
  area *= PI;
  printf("%0.10lf\n", area);
  return 0;
}
