#include <bits/stdc++.h>
#pragma comment(linker, "/stack:64000000")
using namespace std;
template <typename first>
inline first abs(const first& a) {
  return a < 0 ? -a : a;
}
template <typename first>
inline first sqr(const first& a) {
  return a * a;
}
const long double PI = 3.1415926535897932384626433832795;
const long double EPS = 1e-9;
const int INF = 1000 * 1000 * 1000;
long double sq(long double r) { return PI * r * r; }
int main() {
  int n;
  cin >> n;
  vector<int> v(n);
  for (int i = 0; i < int(n); i++) cin >> v[i];
  sort((v).begin(), (v).end());
  reverse((v).begin(), (v).end());
  v.push_back(0);
  long double ans = 0.0;
  for (int i = 0; i < n; i += 2) ans += sq(v[i]) - sq(v[i + 1]);
  cout << setprecision(10) << fixed;
  cout << ans << endl;
  return 0;
}
