#include <bits/stdc++.h>
using namespace std;
int main() {
  long n, i, count = 0;
  cin >> n;
  double a[n], j, ans = 0;
  for (i = 0; i < n; i++) {
    cin >> a[i];
  }
  sort(a, a + n);
  for (i = n - 1; i >= 0; i--) {
    if (count % 2 == 0) {
      ans += 3.1415926536 * a[i] * a[i];
      count++;
    } else {
      ans -= 3.1415926536 * a[i] * a[i];
      count++;
    }
  }
  cout << fixed << setprecision(9) << ans << endl;
  return 0;
}
