#include <bits/stdc++.h>
using namespace std;
vector<int> v;
const double PI = acos(-1);
bool Comparer(int x, int y) { return x > y; }
int main() {
  int n, in, ans = 0;
  cin >> n;
  while (n--) {
    cin >> in;
    v.push_back(in);
  }
  sort(v.begin(), v.end(), Comparer);
  for (int i = 0; i < v.size(); i++) {
    if (i % 2 == 0)
      ans += (v[i] * v[i]);
    else
      ans -= (v[i] * v[i]);
  }
  cout << ans * PI;
  return 0;
}
