#include <bits/stdc++.h>
using namespace std;
vector<int> r;
int main() {
  int n, in;
  cin >> n;
  r.push_back(0);
  for (int i = 0; i < n; i++) {
    cin >> in;
    r.push_back(in);
  }
  n++;
  sort(r.begin(), r.end());
  double res = 0;
  for (int i = 1; i < n; i += 2)
    res += r[i] * r[i] * 3.1415926536,
        res -= r[i - 1] * r[i - 1] * 3.1415926536;
  n--;
  double all = r[n] * r[n] * 3.1415926536;
  if (n & 1)
    printf("%.16lf", res);
  else
    printf("%.16lf", all - res);
}
