本题两种做法均可AC。
一、定义四个变量，O(n)时间

#include<iostream>
using namespace std;
int group[1000000];
int main()
{
    int gnum,top,bottom;
    cin>>gnum;
    for(int i=0;i<gnum;i++)
    {
        cin>>group[i];
    }
    cin>>bottom>>top;
    long long moretop=0,morebottom=0,lesstop=0,lessbottom=0;
/*
moretop 所有组中大于上界的人数和
morebottom 所有组中大于下界的人数和
lesstop  所有组中小于上界的人数和
lessbottom 所有组中小于下界的人数和 
*/ 
    for(int i=0;i<gnum;i++)
    {
        if(group[i]<bottom)
        {
            lesstop+=(long long)top-group[i];
            lessbottom+=(long long)bottom-group[i];
        }
        else if(group[i]<top)
        {
            lesstop+=(long long)top-group[i];
            morebottom+=(long long)group[i]-bottom;
        }
        else
        {
            moretop+=(long long)group[i]-top;
            morebottom+=(long long)group[i]-bottom;
        }
    }
//极端情况是每组学生数都是上界或都是下界
//moretop>lesstop 过上界的学生数大于小于上界的学生数，也就是总学生数过多，不可以
//morebottom<lessbottom 同上理，总学生数太少，不可以 
    if(moretop>lesstop||morebottom<lessbottom)
    {
        cout<<-1<<endl;
    }
//最少调整应取max，因为既要>=下界，又要<=上界 
    else
    {
        cout<<max(lessbottom,moretop)<<endl;
    }
    return 0; 
}

二、模拟，时间复杂度略大，约为O(nlogn)

#include<iostream>
#include<algorithm>
using namespace std;
int group[1000000];
int main()
{
    int gnum;
    cin>>gnum;

    int top,bottom; 
    for(int i=0;i<gnum;i++)
        cin>>group[i];
    sort(group,group+gnum);
    cin>>bottom>>top;
    int curl=0,curr=gnum-1;
    long long ans=0;
    //首先将大于上界或小于下界的学生互补 
    while(curl<curr)
    {
        if(group[curl]>=bottom||group[curr]<=top)   break;
        if(group[curr]-top>bottom-group[curl])
        {
            ans+=(long long)bottom-group[curl];
            group[curr]-=bottom-group[curl];
            group[curl]=bottom;
            curl++;
        }
        else
        {
            ans+=(long long)group[curr]-top;
            group[curl]+=group[curr]-top;
            group[curr]=top;
            curr--;
        }
    }
    sort(group,group+gnum);
//若仍然有小于下界的学生，那么此时所有组均<=上界，再用大于下界的填小于下界的 
    if(group[0]<bottom)
    {
        curl=0;curr=gnum-1;
        while(curl<curr)
        {
            if(group[curl]>bottom||group[curr]<bottom)  break;
            if(group[curr]-bottom>bottom-group[curl])
            {
                ans+=(long long)bottom-group[curl];
                group[curr]-=bottom-group[curl];
                group[curl]=bottom;
                curl++;
            }
            else
            {
                ans+=(long long)group[curr]-bottom;
                group[curl]+=group[curr]-bottom;
                group[curr]=bottom;
                curr--;
            }
        }
        for(int i=0;i<gnum;i++)
        {
            if(group[i]<bottom||group[i]>top)
            {
                ans=-1;
                break;
            }
        }
    }
//若仍然有大于上界的学生，那么此时所有组均>=下界，再用大于上界界的填小于上界界的  
    else
    {
        curl=0,curr=gnum-1;
        while(curl<curr)
        {
            if(group[curl]>top||group[curr]<=top)   break;
            if(group[curr]-top>top-group[curl])
            {
                ans+=(long long)top-group[curl];
                group[curr]-=top-group[curl];
                group[curl]=top;
                curl++;
            }
            else
            {
                ans+=(long long)group[curr]-top;
                group[curl]+=group[curr]-top;
                group[curr]=top;
                curr--;
            }
        }
        for(int i=0;i<gnum;i++)
        {
            if(group[i]<bottom||group[i]>top)
            {
                ans=-1;
                break;
            }
        }
    }
    cout<<ans<<endl;
    return 0;
}