#include <bits/stdc++.h>
using namespace std;
double pi = 3.14159265359;
int n, r[105], s1, s2;
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> r[i];
  }
  sort(r + 1, r + 1 + n);
  reverse(r + 1, r + 1 + n);
  for (int i = 1; i <= n; i++) {
    if (i % 2 == 1) {
      s1 += r[i] * r[i];
    } else
      s2 += r[i] * r[i];
  }
  double ans = (s1 - s2) * pi;
  cout.precision(11);
  cout << fixed << ans;
}
