#include <bits/stdc++.h>
using namespace std;
int A[100005];
int main() {
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> A[i];
  sort(A + 1, A + n + 1);
  int ans = 0;
  int k = 1;
  for (int i = n; i >= 1; i--) ans += (k * A[i] * A[i]), k *= -1;
  printf("%.10f\n", (double)ans * 3.1415926536);
  return 0;
}
