#include <bits/stdc++.h>
using namespace std;
bool cmp(int x, int y) { return x > y; }
int main() {
  double sum;
  int n, r[110];
  while (~scanf("%d", &n)) {
    sum = 0;
    for (int i = 0; i < n; i++) scanf("%d", &r[i]);
    sort(r, r + n, cmp);
    int tmp = 1;
    for (int i = 0; i < n; i++) {
      sum += tmp * r[i] * r[i];
      tmp *= -1;
    }
    printf("%.15f\n", sum * 3.141592653589793);
  }
  return 0;
}
