func_code_string: #include <bits/stdc++.h>   using namespace std;   double pi = 3.1415926536;   int main() {     int n;     cin >> n;     int r[n];     for (int i = 0; i < n; i++) cin >> r[i];     double area = 0;     sort(r, r + n);     for (int i = n - 1; i >= 0; i -= 2) {       if (i > 0)         area += (r[i] * r[i] - r[i - 1] * r[i - 1]);       else         area += r[0] * r[0];     }     area *= pi;     printf( %0.10lf , area);     return 0;   }   