Can anyone tell me, what's wrong with this merge sort code! I'm new to programming and can't figure it out. Thank you in advance!

#include <iostream>
using namespace std;
void merge(int arr[], int p, int q, int r){
	
	int a = q - p + 1;
	int b = r - q;
	
	int left[a];
	int right[b];
	
	for(int i=0;i<a;i++){
		left[i] = arr[p + i];
	}
	for(int j=0;j<b;j++){
		right[j] = arr[q + j + 1];
	}
	
	int i=0;
	int j=0;
	int k=1;
	
	while(i < a && j < b){
		if(left[i] <= right[j]){
			arr[k] = left[i];
			i++;
			k++;
		}else{
			arr[k] = right[j];
			j++;
			k++;
		}
	}
	
	while(i < a){
		arr[k] = left[i];
		i++;
		k++;
	}
	
	while(j < b){
		arr[k] = right[j];
		j++;
		k++;
	}
}
void mergeSort(int arr[], int p, int r){
	if(p < r){
		int q = p + (r-p)/2;
		
		mergeSort(arr, p, q);
		mergeSort(arr, q+1, r);
		merge(arr, p, q, r);
		
	}
}

int main() {

    int arr[] = {12, 11, 13, 5, 6, 7}; 
    int arr_size = sizeof(arr)/sizeof(arr[0]); 
  
    printf("Given array is \n"); 
    for(int i=0;i<arr_size;i++){
    	cout<<arr[i]<<" ";
    }
  
    mergeSort(arr, 0, arr_size - 1); 
  
    printf("\nSorted array is \n"); 
     for(int i=0;i<arr_size;i++){
    	cout<<arr[i]<<" ";
    }
    return 0; 
}