def commonChild(s1, s2):
    n=len(s1)
    g=len(s2)
    m=[]
    for i in range(n+1):
        m.append([])
        for _ in range(g+1):
            m[i].append(0)
    for i in range(n):
        for j in range(g):
            if s1[i]==s2[j]:
                m[i+1][j+1]=max(m[i][j]+1,m[i+1][j-1])
                continue
            m[i+1][j+1]=max(m[i+1][j],m[i][j+1])
    return m[-1][-1]