#include <bits/stdc++.h>
using namespace std;
int main() {
  long long t;
  cin >> t;
  long long a[t], sum;
  for (int i = 0; i < t; ++i) {
    cin >> a[i];
  }
  sort(a, a + t);
  if (t % 2 == 0) {
    sum = 0;
    for (int i = 0; i < t - 1; ++i) {
      sum += a[i + 1] * a[i + 1] - a[i] * a[i];
      i++;
    }
  } else {
    sum = a[0] * a[0];
    for (int i = 1; i < t; i++) {
      sum += a[i + 1] * a[i + 1] - a[i] * a[i];
      i++;
    }
  }
  cout << sum * 3.14159265;
}
