I think you might be misunderstanding the statement. You don’t need to sum anything; instead, you should compare each element with every element to its right, and if the number is greater than all the elements to its right, then it is a leader number.

Example:

17 is a leader number because 17 > 4, 3, 5, 2.

4 is not a leader number because, although 4 > 3 and 2, it is less than 5.