#include <bits/stdc++.h>
const long long inf = 1e18 + 44;
const int MAX = 3e5 + 9;
const long long int MOD = 1e9 + 7;
const int TOT_PRIMES = 1e6 + 9;
const int MAX_A = 71;
const int LN = 20;
const double eps = 1e-1;
double const PI = 3.1415926535897931;
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n, m;
  vector<int> rd;
  cin >> n;
  if (n % 2) rd.push_back(0);
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    rd.push_back(x);
  }
  sort(rd.begin(), rd.end());
  long long int ar = 0;
  for (int i = 0; i < (int)(rd.size()); i++) {
    if (i % 2 == 0)
      ar = ar - (rd[i]) * (rd[i]);
    else
      ar = ar + (rd[i]) * (rd[i]);
  }
  cout << setprecision(17) << (double)ar * PI << endl;
}
