#include <bits/stdc++.h>
using namespace std;
const double PI = 4 * atan(1);
int main() {
  int n;
  double Ans = 0;
  int R[1000];
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> R[i];
  sort(R, R + n + 1);
  for (int i = n; i >= 1; i -= 2) {
    if (i == 1)
      Ans += (PI * (R[i] * R[i] - 0 * 0));
    else
      Ans += (PI * (R[i] * R[i] - R[i - 1] * R[i - 1]));
  }
  cout << Ans << endl;
  return 0;
}
