#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int r[n];
  for (int i = 0; i < n; i++) {
    cin >> r[i];
  }
  sort(r, r + n);
  if (n % 2) {
    int A = pow(r[0], 2);
    for (int i = 2; i < n; i += 2) {
      A += (pow(r[i], 2) - pow(r[i - 1], 2));
    }
    cout << setprecision(10) << (3.1415926536 * A);
  } else {
    int A = 0;
    for (int i = 1; i < n; i += 2) {
      A += (pow(r[i], 2) - pow(r[i - 1], 2));
    }
    cout << setprecision(10) << (3.1415926536 * A);
  }
}
