#include <bits/stdc++.h>
const int INF = 2 * 1e9;
const long long int MOD = 1000003;
using namespace std;
void solve() {
  long long int n;
  cin >> n;
  vector<long long int> a(n);
  for (long long int(i) = 0; (i) < (n); (i)++) cin >> a[i];
  sort((a).begin(), (a).end());
  reverse((a).begin(), (a).end());
  double s = 0;
  for (long long int i = 0; i < n; i += 2) {
    if (i != n - 1)
      s += (a[i] * a[i] - a[i + 1] * a[i + 1]) * 3.14159265358979323846;
    else
      s += a[i] * a[i] * 3.14159265358979323846;
  }
  cout << s;
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cout.fixed;
  cout.precision(12);
  solve();
  return 0;
}
