#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1);
int main() {
  int n;
  cin >> n;
  vector<int> v;
  for (int i = 0, _e(n); i < _e; i++) {
    int x;
    cin >> x;
    v.push_back(x);
  }
  int s = v.size();
  bool innerred;
  if (s % 2 == 0)
    innerred = false;
  else
    innerred = true;
  sort(v.begin(), v.end());
  double ans = 0.0;
  int i;
  if (innerred == true)
    i = 0;
  else
    i = 1;
  while (i < v.size()) {
    ans += pi * (v[i] * v[i]);
    if (i - 1 >= 0) ans -= pi * (v[i - 1] * v[i - 1]);
    i += 2;
  }
  printf("%.15lf\n", ans);
  return 0;
}
