#include <bits/stdc++.h>
using namespace std;
int r[110];
int main() {
  int n;
  double res = 0, pi = 3.1415926536;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> r[i];
  sort(r, r + n);
  for (int i = 0; i < n; i++) {
    if (i % 2 == 0) res += pi * pow(r[i] + 0.0, 2);
    if (i % 2 != 0) res -= pi * pow(r[i] + 0.0, 2);
  }
  if (n % 2 == 0)
    cout << -1 * res;
  else
    cout << res;
  return 0;
}
