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