#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1);
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (auto &c : a) cin >> c;
  sort(a.rbegin(), a.rend());
  double ans = 0;
  for (int i = 0; i < n; i++) {
    if (i & 1)
      ans -= a[i] * a[i] * PI;
    else
      ans += a[i] * a[i] * PI;
  }
  cout << fixed << setprecision(10);
  cout << ans << endl;
  return 0;
}
