#include <bits/stdc++.h>
const int INF = 0x7fffffff;
const int maxn = 100000010;
const long long mod = 2147493647;
const double eps = 1e-4;
const double PI = 3.1415926535898;
using namespace std;
int a[1010];
bool cmp(int a, int b) { return a > b; }
int main(void) {
  int n;
  while (~scanf("%d", &n)) {
    memset(a, 0, sizeof(a));
    for (int i = 0; i < n; i++) scanf("%d", &a[i]);
    sort(a, a + n, cmp);
    int flag = 0;
    double sum = 0;
    for (int i = 0; i < n; i += 2) {
      sum += PI * a[i] * a[i] - PI * a[i + 1] * a[i + 1];
    }
    printf("%.10lf\n", sum);
  }
  return 0;
}
