#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int r[105];
  int i;
  double pi = acos(-1.0);
  for (i = 0; i < n; i++) {
    cin >> r[i];
  }
  sort(r, r + n);
  int s = 0;
  for (i = 0; i < n; i++) {
    s = r[i] * r[i] - s;
  }
  printf("%.6lf\n", s * pi);
  return 0;
}
