#include <bits/stdc++.h>
using namespace std;
int main() {
  double ans = 0.0, radii;
  vector<double> v;
  int n;
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    scanf("%lf", &radii);
    v.push_back(radii);
  }
  sort(v.begin(), v.end());
  reverse(v.begin(), v.end());
  for (int i = 0; i < v.size(); i++) {
    if (i % 2 == 0) {
      ans += (acos(-1) * v[i] * v[i]);
    } else {
      ans -= (acos(-1) * v[i] * v[i]);
    }
  }
  printf("%0.8lf\n", ans + 1e-10);
  return 0;
}
