count inversion

using namespace std;
int marge(int arr[],int l,int mid,int r){
    int inv = 0;
    int n1 = mid-l+1;
    int n2 = r-mid;
    int a[n1];
    int b[n2];
    for(int i=0;i<n1;i++){
        a[i] = a[l+i];
    }
    for(int i=0;i<n2;i++){
        b[i] = b[mid+i+1];
    }
    int i=0,j=0,k=1;
    while(i<n1 && j<n2){
        if(a[i]<=b[j]){
            arr[k] = a[i];
            k++;
            i++;
        }else{
            arr[k] = b[j];
            inv += n1-i;
            k++;
            j++;
        }
    }
    while(i<n1){
        arr[k] = a[i];
        k++;
        i++;
    }
    while(j<n2){
        arr[k] = b[j];
        k++;
        j++;
    }
    return inv;
}
int margesort(int arr[],int l,int r){
    int inv=0;
    if(l < r){
        int mid = (l+r)/2;
        inv += margesort(arr,l,mid);
        inv += margesort(arr,mid+1,r);
        inv += marge(arr,l,mid,r);
    }
    return inv;
}
int main(){
    int n;
    cout<<"Enter the size of array - ";
    cin>>n;
    int arr[n];
    cout<<"Enter the elements of array - ";
    for(int i=0;i<n;i++){
        cin>>arr[i];
    }
    cout<<"Number of inversion - "<<margesort(arr,0,n-1);
    cout<<endl;
    return 0;
}