#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long n;
  const double PI = 3.141592653589793;
  cin >> n;
  vector<long long> v(n);
  for (long long i = 0; i < n; i++) cin >> v[i];
  sort(v.begin(), v.end());
  long long i = n - 1;
  long long rn = 0;
  while (i >= 0) {
    if (i == 0) {
      rn = rn + (v[i] * v[i]);
    } else {
      long long ro = v[i];
      long long ri = v[i - 1];
      rn += (ro * ro) - (ri * ri);
    }
    i = i - 2;
  }
  double area = PI * rn;
  cout << setprecision(10) << fixed << area;
  return 0;
}
