#include <bits/stdc++.h>
using namespace std;
inline int read() {
  int s = 0;
  bool f = 0;
  char ch = ' ';
  while (!isdigit(ch)) {
    f |= (ch == '-');
    ch = getchar();
  }
  while (isdigit(ch)) {
    s = (s << 3) + (s << 1) + (ch ^ 48);
    ch = getchar();
  }
  return (f) ? (-s) : (s);
}
inline void write(int x) {
  if (x < 0) {
    putchar('-');
    x = -x;
  }
  if (x < 10) {
    putchar(x + '0');
    return;
  }
  write(x / 10);
  putchar((x % 10) + '0');
  return;
}
const int N = 105;
const double Pi = 3.141592653589793238;
int n;
double r[N];
int main() {
  int i;
  double R = 0;
  n = read();
  for (i = 1; i <= n; i++) scanf("%lf", &r[i]);
  sort(r + 1, r + n + 1);
  for (i = n; i >= 1; i -= 2) {
    R += r[i] * r[i];
  }
  for (i = n - 1; i >= 1; i -= 2) {
    R -= r[i] * r[i];
  }
  printf("%.10lf\n", Pi * R);
  return 0;
}
