#include <bits/stdc++.h>
using namespace std;
double r[101];
bool cmp(double const &a, double const &b) { return a > b; }
int main() {
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> r[i];
  sort(r + 1, r + n + 1, cmp);
  double ans = 0;
  for (int i = 1; i <= n; i++) {
    if (i % 2 == 1)
      ans += 3.1415926535898 * r[i] * r[i];
    else
      ans -= 3.1415926535898 * r[i] * r[i];
  }
  cout << ans << endl;
  return 0;
}
