#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, s[101];
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%d", &s[i]);
  sort(s, s + n);
  double area = 0.0;
  for (int i = n - 1; i >= 0; i--) {
    if ((n - i) % 2 == 1)
      area += (double)s[i] * s[i] * 3.141592653589793;
    else
      area -= (double)s[i] * s[i] * 3.141592653589793;
  }
  printf("%.6lf\n", area);
  return 0;
}
