#include <bits/stdc++.h>
using namespace std;
double r[105];
int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%lf", &r[i]);
  sort(r, r + n);
  double ans = 0;
  if (n % 2) {
    ans += acos(-1.0) * r[0] * r[0];
    for (int i = n - 1; i >= 1; i -= 2)
      ans += acos(-1.0) * (r[i] * r[i] - r[i - 1] * r[i - 1]);
  } else {
    for (int i = n - 1; i >= 0; i -= 2)
      ans += acos(-1.0) * (r[i] * r[i] - r[i - 1] * r[i - 1]);
  }
  printf("%.10lf\n", ans);
  return 0;
}
