#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1);
const double eps = 1e-9;
int main() {
  cout.precision(9);
  int n;
  cin >> n;
  double r[n + 1];
  r[0] = 0;
  for (int i = 1; i <= n; i++) cin >> r[i];
  sort(r, r + n + 1);
  double ans = 0;
  for (int i = (n & 1 ? 1 : 2); i <= n; i += 2)
    ans += PI * r[i] * r[i] - PI * r[i - 1] * r[i - 1];
  cout << fixed << ans << endl;
  return 0;
}
