#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, j, n, a[1000], k, s, s1;
  double sum;
  while (cin >> n) {
    for (i = 0; i < n; i++) {
      cin >> a[i];
    }
    sort(a, a + n);
    s = 0;
    s1 = 0;
    for (j = n - 1; j >= 0; j = j - 2) {
      s = s + (a[j] * a[j]);
    }
    for (k = n - 2; k >= 0; k = k - 2) {
      s1 = s1 + (a[k] * a[k]);
    }
    sum = 0;
    sum = (s - s1) * acos(-1);
    cout << fixed << setprecision(10) << sum << endl;
  }
  return (0);
}
