func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     int n;     cin >> n;     int a[n], i;     double ans = 0.0f;     for (i = 0; i < n; i++) cin >> a[i];     sort(a, a + n);     if (n % 2 == 0) {       for (i = 0; i < n; i = i + 2) {         ans = ans + ((a[i + 1] * a[i + 1]) - (a[i] * a[i]));       }     } else {       ans = ans + a[0] * a[0];       for (i = 1; i < n; i = i + 2) {         ans = ans + ((a[i + 1] * a[i + 1]) - (a[i] * a[i]));       }     }     double pi = asin(1) * 2;     cout << fixed << setprecision(10) << ans * pi <<   n ;     return 0;   }   