#include <bits/stdc++.h>
using namespace std;
int main() {
  double a[105];
  int n, i;
  cin >> n;
  for (i = 0; i < n; i++) {
    cin >> a[i];
  }
  sort(a, a + n, greater<double>());
  double ans = 0;
  i = 0;
  while (i < n) {
    if (i != n - 1) {
      ans += 3.1415926536 * (a[i] * a[i] - a[i + 1] * a[i + 1]);
    } else {
      ans += 3.1415926536 * (a[i] * a[i]);
    }
    i += 2;
  }
  cout << fixed << setprecision(10) << ans;
}
