func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     int n;     cin >> n;     vector<int> a(n);     for (int i = 0; i < n; ++i) {       cin >> a[i];     }     vector<int> d(5000, 0);     for (int i = 0; i < n; ++i) {       for (int j = i + 1; j < n; ++j) {         ++d[abs(a[i] - a[j])];       }     }     vector<int> c(5000);     c[4999] = 0;     for (int i = 4998; i >= 0; --i) {       c[i] = c[i + 1] + d[i + 1];     }     long long good = 0;     for (int d1 = 1; d1 < 5000; ++d1) {       for (int d2 = 1; d2 < 5000 - d1; ++d2) {         good += static_cast<long long>(d[d1]) * d[d2] * c[d1 + d2];       }     }     long long all = n * (n - 1) / 2;     all = all * all * all;     cout.precision(10);     cout << fixed          << static_cast<long double>(good) / static_cast<long double>(all);     return 0;   }   