#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1.0);
int main() {
  int i, a[101], n;
  double sum = 0;
  scanf("%d", &n);
  for (i = 0; i < n; i++) {
    scanf("%d", &a[i]);
  }
  sort(a, a + n);
  for (i = 0; i < n; i++) {
    if (i % 2 == 0)
      sum -= (pi * (a[i] * a[i]));
    else
      sum += (pi * (a[i] * a[i]));
  }
  printf("%.15lf", fabs(sum));
  return 0;
}
