Учитель сама придумывает задания

#include <iostream>

#include <cmath>

#include <algorithm>

#pragma GCC optimize("Ofast")

#define ll long long

#define ld long double

using namespace std;

signed main() {

  ll n;

  cin >> n;

  ll a[n];

  for(ll i = 0; i < n; i++)

      cin >> a[i];

  ld sum = 0, sumkv = 0, sum6 = 0;

  ll prod = 1;

  for(ll i = 0; i < n; i++){

      sum += a[i];

      prod *= a[i];

      if(i < 6)

          sum6 += a[i];

      sumkv += pow(a[i],2);

  }

  ll sum_k = 0;

  ll k1,k2;

  cin >> k1 >> k2;

  for(ll i = k1-1; i < k2; i++)

      sum_k+=a[i];

  ld av = sum/n;

  ld av_s;

  ld sum_s = 0;

  ll s1,s2;

  cin >> s1 >> s2;

  for(ll i = s1-1;i < s2; i++)

      sum_s += a[i];

  av_s = sum_s/(s2-s1 + 1);

  cout << "a) " << sum << "\n";

   cout << "б) " << prod << "\n";

   cout << "в) " << sumkv << "\n";

   cout << "г) " << sum6 << "\n";

   cout << "д) " << sum_k << "\n";

   cout << "е) " << av << "\n";

   cout << "ж) " << av_s << "\n";

}