#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, a[100], i, z = 0;
  cin >> n;
  for (i = 0; i < n; i++) cin >> a[i];
  sort(a, a + n);
  for (i = n - 1; i >= 0; i -= 2) {
    if (i == 0)
      z += a[i] * a[i];
    else
      z += (a[i] * a[i] - a[i - 1] * a[i - 1]);
  }
  cout << z * 3.1415926536;
  return 0;
}
