#include <bits/stdc++.h>
using namespace std;
const long double Pi = 3.14159265358979323846264338327950288419716939937510;
int n, x;
vector<long double> r;
long double res;
double sq(long double r) { return Pi * r * r; }
int main() {
  cin >> n;
  for (int i = 0; i < (n); ++i) {
    cin >> x;
    r.push_back(x);
  }
  r.push_back(0);
  sort(r.begin(), r.end());
  reverse(r.begin(), r.end());
  for (int i = 0; i < r.size(); i += 2) {
    res += sq(r[i]) - sq(r[i + 1]);
  }
  cout.precision(24);
  cout << res;
  return 0;
}
