You are given two strings `word1` and `word2`. Merge the strings by adding letters in alternating order, starting with `word1`. If a string is longer than the other, append the additional letters onto the end of the merged string.

Return the merged string.

Example 1:

    Input: word1 = "abc", word2 = "pqr"

    Output: "apbqcr"

    Explanation: The merged string will be merged as so:

    word1:  a   b   c

    word2:    p   q   r

    merged: a p b q c r

Example 2:

    Input: word1 = "ab", word2 = "pqrs"

    Output: "apbqrs"

    Explanation: Notice that as word2 is longer, "rs" is appended to the end.

    word1:  a   b

    word2:    p   q   r   s

    merged: a p b q   r   s

Example 3:

    Input: word1 = "abcd", word2 = "pq"

    Output: "apbqcd"

    Explanation: Notice that as word1 is longer, "cd" is appended to the end.

    word1:  a   b   c   d

    word2:    p   q

    merged: a p b q c   d

Constraints:

- `1 <= word1.length, word2.length <= 100`
- `word1` and `word2` consist of lowercase English letters.