#include <bits/stdc++.h>
using namespace std;
const long long MX = 1e5 + 9;
const long double pi = acos(-1);
bool cmp(int a, int b) { return a > b; }
int a[MX];
int main() {
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  sort(a + 1, a + n + 1);
  long double ans = 0, c = 0;
  for (int i = n; i > 0; i -= 2) {
    ans += pi * a[i] * a[i] - (pi * a[i - 1] * a[i - 1]);
  }
  cout << fixed << setprecision(10) << ans;
  return 0;
}
