PRDTPAIN - COOK135B

it seems that my program is correct and it successfully runs all test cases but still i get wrong answer can anyone help me
Que link :- Contest Page | CodeChef

here is the code

#include<bits/stdc++.h>
using namespace std;

//@Himanshu_nobee

#define f(i,s,e) for(long long i=s;i<e;i++)
#define endl "\n"
#define ll long long

int t;
int main()
{
    //@Himanshu_nobee
    ios_base::sync_with_stdio(false);
    cin.tie(NULL); cout.tie(NULL);
    cin>>t;
    while(t--)
    {
        //@Himanshu_nobee
        ll n,q,x=0,y,sum=0;
        cin>>n;
        ll a[n],b[n-1];
        f(i,0,n)cin>>a[i];
       // f(i,0,n-1) b[i]+=a[i+1]-a[i];
        for(int i=0;i<n-2;i++)
        {
            int k=i+1;
            for(int j=i+2;j<n;j++)
            {
                ll ans=(a[k]-a[i])*(a[j]-a[k]);
                while((k<j-1)&&(ans<(a[k+1]-a[i])*(a[j]-a[k+1])))
                {
                ans=(a[k+1]-a[i])*(a[j]-a[k+1]);
                    k++;
                }
            sum+=ans;
            }
        }
        cout<<sum<<endl;

    }

}

I did using some other method but same issue as you.
It is giving WA and I don’t know why