#include <bits/stdc++.h>
using namespace std;
const double pi = 3.141592653589793;
int main() {
  int n;
  cin >> n;
  int s[n];
  for (int i = 0; i < n; i++) cin >> (s[i]);
  double sum;
  sort(s, s + n, greater<int>());
  for (int i = 0; i < n; i++) {
    if (i % 2 == 0)
      sum += pi * (s[i] * s[i]);
    else
      sum -= pi * (s[i] * s[i]);
  }
  cout << setprecision(10) << fixed << sum << endl;
  return 0;
}
