#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int r[1010] = {0};
  int i;
  int x1;
  for (i = 0; i < n; i++) {
    cin >> x1;
    r[x1] = 1;
  }
  long double a = 0;
  double x = 1;
  double y;
  for (i = 1000; i >= 0; i--) {
    if (r[i] != 0) {
      y = double(i);
      a = a + (x * y * y);
      x = x * -1;
    }
  }
  long double den = 3.1415926536;
  long double area = (den)*a;
  cout << setprecision(11) << area << endl;
  return 0;
}
