def commonChild(s1, s2):
    dp = [[0] * (len(s2) + 1) for _ in range(len(s1) + 1)]
    
    for i in range(1, len(s1) + 1):
        for j in range(1, len(s2) + 1):
            if s1[i - 1] == s2[j - 1]:
                dp[i][j] = dp[i - 1][j - 1] + 1
            else:
                dp[i][j] = max(dp[i - 1][j], dp[i][j - 1])
    return dp[len(s1)][len(s2)]