Unable to understand the logic behind the solution[FrogRiverOne]

I m unable to understand what is logic behind the solution for Codility FrogRiverOne here https://codility.com/demo/take-sample-test/frog_river_one

Task description

A small frog wants to get to the other side of a river. The frog is initially located on one bank of the river (position 0) and wants to get to the opposite bank (position X+1). Leaves fall from a tree onto the surface of the river.

You are given an array A consisting of N integers representing the falling leaves. A[K] represents the position where one leaf falls at time K, measured in seconds.

The goal is to find the earliest time when the frog can jump to the other side of the river. The frog can cross only when leaves appear at every position across the river from 1 to X (that is, we want to find the earliest moment when all the positions from 1 to X are covered by leaves). You may assume that the speed of the current in the river is negligibly small, i.e. the leaves do not change their positions once they fall in the river.

For example, you are given integer X = 5 and array A such that:

  A[0] = 1
  A[1] = 3
  A[2] = 1
  A[3] = 4
  A[4] = 2
  A[5] = 3
  A[6] = 5
  A[7] = 4

In second 6, a leaf falls into position 5. This is the earliest time when leaves appear in every position across the river.

Write a function:

def solution(X, A)

that, given a non-empty array A consisting of N integers and integer X, returns the earliest time when the frog can jump to the other side of the river.

If the frog is never able to jump to the other side of the river, the function should return −1.

For example, given X=5 and array A such that:

A[0] = 1
A[1] = 3
A[2] = 1
A[3] = 4
A[4] = 2
A[5] = 3
A[6] = 5
A[7] = 4

the function should return 6, as explained above.

Assume that:

N and X are integers within the range [1 . 100,000] each element of array A is an integer within the range (1 . X). Complexity:

expected worst-case time complexity is O(N); expected worst-case space complexity is O(X) (not counting the storage required for input arguments).

Solution--

Input arguments to Function - (2,[2,2,2,2,2]) and (5, [1, 3, 1, 4, 2, 3, 5, 4])

def solution(X,A):

    covered = 0
    covered_a = [-1]*X

    for index,element in enumerate(A):
        if covered_a[element-1] == -1:

            covered_a[element-1] = element
            covered += 1
            if covered == X:
                return index
    return -1

I want to understand what is the logic behind creating an boolean array and substracting 1 element wise from the input array A

Solution 1:

It's because you want to "flag" all the numbers you've seen so far. So it begins with all of them on 'False' because you haven't seen any of them yet.