#include <bits/stdc++.h>
using namespace std;
int dx[] = {0, 1, 0, -1, -1, -1, 1, 1};
int dy[] = {1, 0, -1, 0, -1, 1, -1, 1};
int n, cir[105], i, ans;
int main() {
  cin >> n;
  for ((n % 2) ? i = 1, n++ : i = 0; i < n; ++i) cin >> cir[i];
  sort(cir, cir + n);
  for (i = n - 1; i > 0; i -= 2) ans += pow(cir[i], 2) - pow(cir[i - 1], 2);
  cout << setprecision(10) << fixed << (double)(ans * 3.1415926536) << endl;
}
