func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     vector<int> vec;     double ans = 0;     int n;     cin >> n;     vec.resize(n);     for (int i = 0; i < n; cin >> vec[i++])       ;     sort(vec.rbegin(), vec.rend());     for (int i = 0; i < n; i += 2) ans += vec[i] * vec[i];     for (int i = 1; i < n; i += 2) ans -= vec[i] * vec[i];     ans *= 3.1415926536;     cout << fixed << setprecision(11) << ans << endl;     return 0;   }   