Four with sum 0 - SPOJ

#include<bits/stdc++.h>
using namespace std;
#define maxx 10000001
int mp[maxx],mp2[maxx];
//--------------upper bound function----------------
int upper_boundd(int n,int hi)
{
    int lo=0,ans=0;
    while(lo<=hi)
    {
        int mid=(lo+hi)/2;
        if(mp[mid]>=n)
        {
            if(mp[mid]==n)
            {
                ans=mid;
                lo=mid+1;          //pushing mean upward
            }
            else
            {
                hi=mid-1;
            }
        }
        else
        {
            lo=mid+1;
        }
    }
    return ans;
}
//-------------------main function-------------
int main()
{
    int n,A[4008],B[4008],C[4008],D[4008];
    while(scanf("%d",&n)==1)
    {
        for(int nn=1;nn<=n;nn++)
        {
            scanf("%d %d %d %d",&A[nn],&B[nn],&C[nn],&D[nn]);
        }
//-----------first 1/2--------------
        int mp_indx=0;
        for(int i=1;i<=n;i++)
        {
            for(int j=1;j<=n;j++)
            {
                int sum=A[i]+B[j];
                mp[mp_indx]=sum;
                mp_indx++;
            }
        }
        sort(mp,mp+mp_indx);
//-----------second 1/2--------------
        int mp2_indx=0;
        for(int i=1;i<=n;i++)
        {
            for(int j=1;j<=n;j++)
            {
                int sum=D[i]+C[j];
                sum=sum*(-1);
                mp2[mp2_indx]=sum;
                mp2_indx++;
            }
        }
        sort(mp2,mp2+mp2_indx);
//-------- compare two arrays of sum---------------
        int ans=0;
        for(int ii=0;ii<mp2_indx;)
        {
            int val=mp2[ii];
            int c=0;
            while(mp2[ii]==val && ii<mp2_indx)
            {
                c++;
                ii++;
            }
            int up=upper_boundd(val,mp_indx-1);
            int jj=up;
            while(mp[jj]==val)
            {
                jj--;
                if(jj<0)    break;
            }
            ans=ans+(c* (up-jj));
        }
        printf("%d\n",ans);
    }
    return 0;
}