#include <bits/stdc++.h>
const unsigned int M = 1000000007;
long long power(long long x, long long y) {
  long long res = 1;
  while (y > 0) {
    if (y & 1) res = x * res;
    y = y >> 1;
    x = x * x;
  }
  return res;
}
long long power(long long x, long long y, long long mod) {
  long long res = 1;
  while (y > 0) {
    if (y & 1) res = x * res % mod;
    y = y >> 1;
    x = x * x % mod;
  }
  return res;
}
using namespace std;
void nik() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
}
signed main() {
  nik();
  long long test = 1;
  while (test--) {
    long long n;
    cin >> n;
    long long a[n];
    for (long long i = 0; i < n; i++) cin >> a[i];
    sort(a, a + n);
    double ans = 0.00000;
    for (long long i = n - 1; i >= 0; i -= 2) {
      if (i != 0)
        ans += 3.141592653589793238 * (a[i] * a[i] - a[i - 1] * a[i - 1]);
      else
        ans += 3.141592653589793238 * a[i] * a[i];
    }
    cout << fixed << setprecision(9) << ans << "\n";
  }
  return 0;
}
