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