コーディングテスト練習サイトCodilityの「FrogRiverOne」という問題について、問題の理解とコードがなぜあるケースでincorrectになるのかわからず困っています。

FrogRiverOne coding task - Learn to Code - Codility
上記のFrogRiverOneについて日本語で書かれた記事

以下問題文を原文引用しました。翻訳もかけてみたのですが、問題文が理解できないです。

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.

例を満たすように以下のコードを書きましたが、テストケース(2, [2, 2, 2, 2, 2])の時に自分のコードだと0が返され、正しくは-1だと結果が出ました。
修正のため、アドバイスをいただきたいです。

def solution(X, A):
    result = 0
    if X not in A:
        result = -1
    else:
        result = A.index(X)
return result