#include <bits/stdc++.h>
using namespace std;
set<int> ist;
map<string, int> msi;
map<string, string> mss;
map<int, string> mis;
map<int, int> mii;
vector<int> v;
vector<pair<int, int> > vv;
int cc[] = {1, -1, 0, 0};
int rr[] = {0, 0, 1, -1};
void LIFT_OFF_FALCON_MA20_D1_D2() {
  int t;
  int n;
  cin >> n;
  int ahm[n + 1];
  for (int i = 1; i <= n; i++) {
    cin >> ahm[i];
  }
  sort(ahm + 1, ahm + n + 1);
  double ans = 0;
  if (n % 2 == 1) {
    for (int i = 1; i <= n; i++) {
      if (i == 1) {
        ans = ans + (ahm[i] * ahm[i]);
      } else if (i % 2 == 1) {
        ans = ans + ((ahm[i] * ahm[i]) - (ahm[i - 1]) * ahm[i - 1]);
      }
    }
  } else if (n % 2 == 0) {
    for (int i = 1; i <= n; i++) {
      if (i % 2 == 0) {
        ans = ans + ((ahm[i] * ahm[i]) - (ahm[i - 1]) * ahm[i - 1]);
      }
    }
  }
  ans = ans * 3.1415926536;
  cout << fixed << setprecision(10) << ans << endl;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  LIFT_OFF_FALCON_MA20_D1_D2();
  return 0;
}
