int _getMissingNumber(int a[], int n)
{

    int total = (n + 1) * (n + 2) / 2;
    for (int i = 0; i < n; i++)
        total -= a[i];
    return total;
}

int main()
{

    int nums[] = {1,2,3,5};

    int missing = _getMissingNumber(nums,sizeof(nums)/sizeof(int));

    printf("%d\n",missing);

    return 0;
}