#include <bits/stdc++.h>
using namespace std;
const double pi = 3.1415926536;
int main() {
  int n;
  cin >> n;
  int c[n];
  for (int i = 0; i < n; i++) {
    cin >> c[i];
  }
  sort(c, c + n);
  double ans = 0.0;
  int cont = 0;
  int anterior = 1001;
  for (int i = n - 1; i >= 0; i--) {
    if (c[i] != anterior) {
      if (cont % 2 == 0) {
        ans += pi * ((double)(c[i] * c[i]));
        cont++;
        anterior = c[i];
      } else {
        ans -= pi * ((double)(c[i] * c[i]));
        cont++;
        anterior = c[i];
      }
    }
  }
  printf("%.6f", ans);
  return 0;
}
