#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  while (cin >> n) {
    int a[n];
    double area = 0;
    for (int i = 0; i < n; i++) {
      cin >> a[i];
    }
    sort(a, a + n);
    if (n % 2 == 0) {
      for (int i = 1; i < n; i += 2) {
        area +=
            3.14159265359 * a[i] * a[i] - 3.14159265359 * a[i - 1] * a[i - 1];
      }
    } else {
      for (int i = 0; i < n; i += 2) {
        if (i == 0)
          area += 3.14159265359 * a[i] * a[i];
        else
          area +=
              3.14159265359 * a[i] * a[i] - 3.14159265359 * a[i - 1] * a[i - 1];
      }
    }
    printf("%.10lf\n", area);
  }
}
