Merge Strings Alternately

class Solution:
    def mergeAlternately(self, word1: str, word2: str) -> str:
        s1 = 0
        s2 = 0
        out = []
        while s1 < len(word1) or s2 < len(word2):
            if s1 < len(word1):
                out.append(word1[s1])
            if s2 < len(word2):
                out.append(word2[s2])
            s1 = s1 + 1
            s2 = s2 + 1
        return ''.join(out)