#include <bits/stdc++.h>
using namespace std;
int main() {
  const double pi = 3.14159265358979;
  int n, i;
  double temp, s1 = 0., s2 = 0.;
  vector<double> r;
  cin >> n;
  for (i = 0; i < n; i++) {
    cin >> temp;
    r.push_back(temp);
  }
  sort(r.begin(), r.end());
  if (n % 2 == 0) {
    for (i = 0; i <= (n - 1) / 2; i++)
      s1 += pi * r[n - 1 - (2 * i)] * r[n - 1 - (2 * i)];
    for (i = 0; i <= (n - 1) / 2; i++)
      s2 -= pi * r[n - 1 - (2 * i + 1)] * r[n - 1 - (2 * i + 1)];
  } else {
    for (i = 0; i <= (n - 1) / 2; i++)
      s1 += pi * r[n - 1 - (2 * i)] * r[n - 1 - (2 * i)];
    for (i = 0; i < (n - 1) / 2; i++)
      s2 -= pi * r[n - 1 - (2 * i + 1)] * r[n - 1 - (2 * i + 1)];
  }
  cout << fixed << setprecision(10) << s1 + s2;
  return 0;
}
