#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1);
int main() {
  int n;
  cin >> n;
  double ans = 0;
  bool turn = true;
  double r[n];
  for (int i = 0; i < n; i++) {
    cin >> r[i];
  }
  sort(r, r + n);
  reverse(r, r + n);
  for (int i = 0; i < n; i++) {
    double S = r[i] * r[i] * PI;
    ans += (turn ? S : -S);
    turn = !turn;
  }
  cout << fixed << setprecision(10) << ans;
}
