#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n;
  cin >> n;
  vector<long double> v(n);
  for (long long i = 0; i < n; i++) cin >> v[i];
  sort(v.begin(), v.end());
  bool f = 0;
  long double ans = 0;
  for (long long i = n - 1; i >= 0; i--) {
    if (f == 0) {
      ans = ans + v[i] * v[i];
    } else {
      ans = ans - v[i] * v[i];
    }
    f = 1 - f;
  }
  ans = ans * 3.1415926536;
  cout << setprecision(10) << ans << endl;
}
