#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  double t = 0;
  cin >> n;
  int i, a[n], b[n];
  for (i = 0; i < n; i++) cin >> a[i];
  sort(a, a + n);
  for (i = 0; i < n; i++) {
    b[i] = a[n - 1 - i];
  }
  for (i = n - 1; i >= 0; i--) {
    if (i % 2 == 0)
      t += (b[i] * b[i]);
    else
      t -= (b[i] * b[i]);
  }
  t *= 3.141592653589793;
  printf("%.15f", t);
  return 0;
}
