func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     long n, i, count = 0;     cin >> n;     double a[n], j, ans = 0;     for (i = 0; i < n; i++) {       cin >> a[i];     }     sort(a, a + n);     for (i = n - 1; i >= 0; i--) {       if (count % 2 == 0) {         ans += 3.1415926536 * a[i] * a[i];         count++;       } else {         ans -= 3.1415926536 * a[i] * a[i];         count++;       }     }     cout << fixed << setprecision(9) << ans << endl;     return 0;   }   