#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  int n;
  cin >> n;
  ll a[n + 1];
  for (int i = 1; i <= n; i++) cin >> a[i];
  sort(a + 1, a + n + 1, greater<ll>());
  ll sum = 0;
  ll total = 0;
  for (int i = 1; i <= n; i++) {
    if (i % 2) {
      sum = sum + a[i] * a[i];
    } else {
      total = total + a[i] * a[i];
    }
  }
  double pi = 3.1415926536;
  double ans = (sum - total) * pi;
  cout << fixed << setprecision(10) << (sum - total) * pi << endl;
}
