#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1.0);
int n, r[1010];
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) scanf("%d", &r[i]);
  sort(r + 1, r + n + 1);
  int k = 0;
  for (int i = n, j = 1; i >= 1; i--, j = -j) k += j * r[i] * r[i];
  printf("%lf\n", pi * k);
  return 0;
}
