#include <bits/stdc++.h>
using namespace std;
vector<int> v, v2, v3;
vector<int>::iterator it1, it2;
int main() {
  int n, t, blue = 0, red;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) {
    int a;
    scanf("%d", &a);
    v.push_back(a);
  }
  sort(v.begin(), v.end());
  if (n & 1) {
    for (int i = 1; i < n; i += 2) {
      blue += (v[i] * v[i]) - (v[i - 1] * v[i - 1]);
    }
  } else {
    blue += v[0] * v[0];
    for (int i = 2; i < n; i += 2) {
      blue += (v[i] * v[i]) - (v[i - 1] * v[i - 1]);
    }
  }
  red = (v[n - 1] * v[n - 1]) - blue;
  double ans = red * 3.1415926536;
  printf("%.10f\n", ans);
}
