Leet Code: Missing Number from Sorted Array of Numbers

 public class MissingNumber {

    public static int missingNumber(int[] nums, int n)
    {
        int sum = ((n + 1) * (n + 2)) / 2; //
        for (int i = 0; i < n; i++)
            sum -= nums[i];
        return sum;
    }

    public static void main(String[] args) {
        int[] a = {1,2,4,5,6};
        System.out.println(missingNumber(a,a.length));
    }
}