Divide into two array of one array element and after that merge them again oposite direction

merge.c

int main() {

    /* Enter your code here. Read input from STDIN. Print output to STDOUT */
    int N, R, K, i, j, index, boysIndex;
    int arr[21];
    int arr2[21];
    int arr3[21];
    int mergeArray[21];

    scanf("%d", &N);

    for (i = 1; i <= N; i++)
    {
        scanf("%d", &R);
        arr[i] = R;
    }

    scanf("%d", &K);

    index = K;
    
    for(i=1; i<=index;i++) {
        arr2[i] = arr[i];
    }

    boysIndex = N - K;


    for(i=1;i<=boysIndex; i++) {
        arr3[i] = arr[i + K];
    }
    
    
    //merge 2 array
    for(j=1;j<=(N - K); j++) {
        //copy element of array 3 to mergeArray
        mergeArray[j] = arr3[j];
    }

    for(j=1;j<=K; j++) {
        // copy element of array 2 to merge array but index start from after last index of first copy element
        mergeArray[j+K] = arr2[j];
    }
    
    
    //merge 2 array
    // i = 1;
    // for(j=1; j<=(N - K); j++) {
    //     mergeArray[i] = arr3[j];
    //     i++;
    // }

    // for (j = 1; j <= K; j++)
    // {
    //     mergeArray[i] = arr2[j];
    //     i++;
    // }

    for(i=1; i<=N; i++) {
        printf("%d ", mergeArray[i]);
    }

    return 0;
    
}