#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[n], i;
  double ans = 0.0f;
  for (i = 0; i < n; i++) cin >> a[i];
  sort(a, a + n);
  if (n % 2 == 0) {
    for (i = 0; i < n; i = i + 2) {
      ans = ans + ((a[i + 1] * a[i + 1]) - (a[i] * a[i]));
    }
  } else {
    ans = ans + a[0] * a[0];
    for (i = 1; i < n; i = i + 2) {
      ans = ans + ((a[i + 1] * a[i + 1]) - (a[i] * a[i]));
    }
  }
  double pi = asin(1) * 2;
  cout << fixed << setprecision(10) << ans * pi << "\n";
  return 0;
}
