#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("Ofast")
double circleA(long long r) { return r * r * 4 * atan(1); }
void solve() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  sort(begin(a), end(a), greater<int>());
  double sum = 0;
  for (int i = 1; i < n;) {
    sum += circleA(a[i - 1]) - circleA(a[i]);
    i += 2;
  }
  if (a.size() % 2) {
    sum += circleA(a[a.size() - 1]);
  }
  cout << setprecision(10) << sum << endl;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  solve();
  return 0;
}
