#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, i, j, a[101], temp;
  double red = 0;
  cin >> n;
  for (i = 0; i < n; i++) cin >> a[i];
  for (i = 0; i < n; i++)
    for (j = 0; j < n; j++)
      if (a[i] > a[j]) {
        temp = a[i];
        a[i] = a[j];
        a[j] = temp;
      }
  for (i = 0; i < n; i++) {
    if (i % 2 == 0)
      red = red + 3.1415926536 * a[i] * a[i];
    else
      red = red - 3.1415926536 * a[i] * a[i];
  }
  cout << red;
  return 0;
}
