#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  long double P = 3.1415926536, a[110], n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    a[i] *= a[i];
    a[i] *= P;
  }
  for (int i = 0; i < n; i++)
    for (int j = 0; j < n - 1; j++)
      if (a[j] > a[j + 1]) swap(a[j], a[j + 1]);
  long double ans = 0;
  for (int i = n - 1; i >= 0; i -= 2)
    if (i == 0)
      ans += a[i];
    else
      ans += a[i] - a[i - 1];
  cout << setprecision(12) << ans;
  return 0;
}
