number of inversions in an array using merge sort

why is the following programme incorrect for finding the number of inversions in an array. To know about inversions google it. for all small inputs that i could devise, i got a correct answer. however for this input of size 100,000 the answer differs from original. The code is:-

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

ll counts=0;

void stitchncount(ll *,ll *,ll *,ll ,ll );
void mergencount(ll *,ll );

int main()
{
    ll n;
    cout<<"enter the number of elements in the array"<<endl;
    cin>>n;
    ll * arr;
    arr=new ll[n];
    for(ll i=0;i<n;i++)
    {
        cin>>arr[i];
    }
    mergencount(arr,n);
    cout<<"the number of inversions are "<<counts<<endl;
    return 0;
}

void stitchncount(ll *arr,ll *arr1,ll *arr2,ll sizedh1,ll sizedh2)
{
    ll sized1,sized2;
    sized1=sizedh1;
    sized2=sizedh2;
    ll i,j,k;
    i=j=k=0;
    while(i<sized1 && j<sized2)
    {
        if(arr1[i]<arr2[j])
        {
            arr[k]=arr1[i];
            i++;
        }
        else
        {
            arr[k]=arr2[j];
            j++;
            counts+=(sized1-i);
        }
        k++;
    }
    if(i==sized1)
    {
        arr[k]=arr2[j];
        j++;
        k++;
    }
    else
    {
        arr[k]=arr1[i];
        i++;
        k++;
    }

}

void mergencount(ll * arr, ll n)
{
    ll sized=n;
    if(sized==1)
    {
        return;
    }
    ll *arr1,*arr2,j=0;
    arr1=arr2=NULL;
    ll sizedh=sized/2;
    arr1=new ll[sizedh];
    arr2=new ll[sized-sizedh];
    for(ll i=0;i<sizedh;i++)
    {
        arr1[i]=arr[i];
        j++;
    }
    for(ll i=0;i<sized-sizedh;i++)
    {
        arr2[i]=arr[j];
        j++;
    }
    mergencount(arr1,sizedh);
    mergencount(arr2,sized-sizedh);
    stitchncount(arr,arr1,arr2,sizedh,sized-sizedh);
}
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