#include <bits/stdc++.h>
using namespace std;
const int N = 1e9 + 7;
void solve() {
  long long int n;
  cin >> n;
  vector<float> v(n);
  for (int i = 0; i < n; i++) cin >> v[i];
  sort(v.begin(), v.end(), greater<float>());
  double ans = 0;
  for (int i = 0; i < v.size(); i++) {
    if (i % 2 == 0)
      ans += 3.1415926536 * (v[i] * v[i]);
    else
      ans -= 3.1415926536 * (v[i] * v[i]);
  }
  cout << setprecision(10) << fixed << ans;
}
int main() {
  long long int t;
  t = 1;
  while (t--) {
    solve();
    cout << endl;
  }
}
