#include <bits/stdc++.h>
using namespace std;
int r[105];
int main() {
  ios_base::sync_with_stdio(false);
  int n;
  double ans;
  cin >> n;
  for (int i = 0; i < n and cin >> r[i]; ++i)
    ;
  sort(r, r + n);
  ans = r[0] * r[0];
  for (int i = n - 1; i > 0; i -= 2) {
    ans += (r[i] + r[i - 1]) * (r[i] - r[i - 1]);
  }
  if (n % 2 == 0) ans -= r[0] * r[0];
  cout << fixed << setprecision(10) << ans * acos(-1);
  return 0;
}
