Language: C++

#include <boost/multiprecision/cpp_int.hpp>
namespace mp = boost::multiprecision;
#include <bits/stdc++.h>
#define _GLIBCXX_DEBUG
using namespace std;
using INT = mp::cpp_int;
using ll = long long;
using vec = vector<ll>;
using vect = vector<double>;
using Graph = vector<vector<ll>>;
#define loop(i, n) for (ll i = 0; i < n; i++)
#define Loop(i, m, n) for (ll i = m; i < n; i++)
#define pool(i, n) for (ll i = n; i >= 0; i--)
#define Pool(i, m, n) for (ll i = n; i >= m; i--)
#define mod 1000000007ll
//#define mod 998244353ll
#define flagcount(bit) __builtin_popcount(bit)
#define flag(x) (1ll << x)
#define flagadd(bit, x) bit |= flag(x)
#define flagpop(bit, x) bit &= ~flag(x)
#define flagon(bit, i) bit &flag(i)
#define flagoff(bit, i) !(bit & (1ll << i))
#define all(v) v.begin(), v.end()
#define low2way(v, x) lower_bound(all(v), x)
#define high2way(v, x) upper_bound(all(v), x)
#define idx_lower(v, x) (distance(v.begin(), low2way(v, x)))  //配列vでx未満の要素数を返す
#define idx_upper(v, x) (distance(v.begin(), high2way(v, x))) //配列vでx以下の要素数を返す
#define idx_lower2(v, x) (v.size() - idx_lower(v, x))         //配列vでx以上の要素数を返す
#define idx_upper2(v, x) (v.size() - idx_upper(v, x))         //配列vでxより大きい要素の数を返す
#define putout(a) cout << a << endl
#define Sum(v) accumulate(all(v), 0ll)
ll ctoi(char c)
{
    if (c >= '0' && c <= '9')
    {
        return c - '0';
    }
    return -1;
}
template <typename T>
string make_string(T N)
{
    string ret;
    T now = N;
    while (now > 0)
    {
        T x = now % 10;
        ret += (char)('0' + x);
        now /= 10;
    }
    reverse(all(ret));
    return ret;
}
template <typename T>
T gcd(T a, T b)
{
    if (a % b == 0)
    {
        return (b);
    }
    else
    {
        return (gcd(b, a % b));
    }
}
template <typename T>
T lcm(T x, T y)
{
    T z = gcd(x, y);
    return x * y / z;
}
template <typename T>
bool primejudge(T n)
{
    if (n < 2)
        return false;
    else if (n == 2)
        return true;
    else if (n % 2 == 0)
        return false;
    double sqrtn = sqrt(n);
    for (T i = 3; i < sqrtn + 1; i++)
    {
        if (n % i == 0)
        {
            return false;
        }
        i++;
    }
    return true;
}
template <typename T>
bool chmax(T &a, const T &b)
{
    if (a < b)
    {
        a = b; // aをbで更新
        return true;
    }
    return false;
}
template <typename T>
bool chmin(T &a, const T &b)
{
    if (a > b)
    {
        a = b; // aをbで更新
        return true;
    }
    return false;
}
//場合によって使い分ける
//const ll dx[4]={1,0,-1,0};
//const ll dy[4]={0,1,0,-1};
const ll dx[8] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dy[8] = {0, 1, 1, 1, 0, -1, -1, -1};
//2次元配列の宣言
//vector<vector<ll>> field(h, vector<ll>(w));
vector<INT> fact(301);    //fact[i]=(i!)
void COMinit()
{
    INT now = 1;
    fact[0] = 1;
    for (long long i = 1; i < 301; i++)
    {
        now *= i;
        fact[i] = now;
    }
}
INT COM(INT n, INT r)
{
    if (n < r)
        return 0;
    if (n < 0 || r < 0)
        return 0;
    if (n == r)
        return 1;
    if (r == 0)
        return 1;
    INT ans = fact[n];
    ans /= fact[r];
    ans /= fact[n - r];
    return ans;
}
int main()
{
    cout << fixed << setprecision(30);
    ll N, A, B;
    cin >> N >> A >> B;
    vector<ll> v(N);
    COMinit();
    loop(i, N) cin >> v[i];
    sort(all(v));
    reverse(all(v));
    double ans1 = 0;
    loop(i, A) ans1 += v[i];
    ans1 /= A;
    putout(ans1);
    /*
    vから大きい順にA個取った集合をSとする
    Sに異なる数が含まれる場合
        A+1個以上取ると平均が減少してしまう
        Sで最小の数をmとして、mがS内にa個、S外にb個あるとすると、
        答えはCOM(a+b,a)
    Sに異なる数が含まれない場合
        その数が全体でX個あるとすると、
        答えはCOM(X,A)+COM(X,A+1)+...COM(X,B)
        (但しs<tならCOM(s,t)=0と仮定する)
    */
    vector<INT> S(A);
    loop(i, A) S[i] = v[i];
    if (S[0] != S[A - 1])
    {
        INT a = 0, b = 0;
        loop(i, A) if (S[i] == S[A - 1]) a++;
        loop(i, N - A) if (v[A + i] == S[A - 1]) b++;
        putout(COM(a + b, a));
        return 0;
    }
    INT X = 0;
    loop(i, N) if (v[i] == v[0]) X++;
    INT ans = 0;
    for (ll i = A; i <= B; i++)
        ans += COM(X, i);
    putout(ans);
    return 0;
}


Compiler output:
a.cc:1:10: fatal error: boost/multiprecision/cpp_int.hpp: No such file or directory
    1 | #include <boost/multiprecision/cpp_int.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.