#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n, k;
  cin >> n;
  set<int> s;
  for (__typeof(n) i = (0) - ((0) > (n)); i != (n) - ((0) > (n));
       i += 1 - 2 * ((0) > (n))) {
    cin >> k;
    s.insert(k);
  }
  int f = 0, se = 1;
  double odd = 0, even = 0;
  for (int c : s) {
    if (se & 1) {
      odd += (3.1415926536 * (c * c - f * f));
    } else {
      even += (3.1415926536 * (c * c - f * f));
    }
    f = c;
    ++se;
  }
  if (s.size() & 1)
    cout << odd;
  else
    cout << even;
  return 0;
}
