#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1.0);
int main() {
  int n;
  cin >> n;
  int a[105];
  double ans = 0.0;
  for (int i = 0; i < n; ++i) cin >> a[i];
  sort(a, a + n);
  if (n & 1) {
    double ant = 0.0;
    for (int i = 0; i < n; ++i) {
      if (i % 2 == 0) ans += (pi * a[i] * a[i] - ant);
      ant = pi * a[i] * a[i];
    }
  } else {
    double ant = 0.0;
    for (int i = 0; i < n; ++i) {
      if (i & 1) ans += (pi * a[i] * a[i] - ant);
      ant = pi * a[i] * a[i];
    }
  }
  cout << ans << endl;
  return 0;
}
