number of inversions in an array using merge sort

general
,
1

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);
}