#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, j, k, l, n, m, x, y, z;
  cin >> n;
  double d = 0, a[10000];
  a[0] = 0.0;
  for (i = 1; i <= n; i++) cin >> a[i];
  sort(a, a + n + 1);
  for (i = n; i >= 1; i -= 2) {
    d = d + 3.14159265359 * (a[i] * a[i] - a[i - 1] * a[i - 1]);
  }
  printf("%.10lf", d);
  return 0;
}
