#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  int sum = 0;
  cin >> n;
  int r[n];
  for (int i = 0; i < n; ++i) {
    cin >> r[i];
  }
  sort(r, r + n);
  sum = r[0] * r[0];
  for (int j = 2; j < n; j += 2) {
    sum += r[j] * r[j] - r[j - 1] * r[j - 1];
  }
  float p = 3.141592653589793;
  if (n % 2 == 0) {
    float a = p * r[n - 1] * r[n - 1] - p * sum;
    cout << a;
  } else {
    float a = p * sum;
    cout << a;
  }
  return 0;
}
