def path_to_friend(network, user_A,user_B, path=None, variable=None):
    if user_A not in network or user_B not in network:
       return None
    if path == None:
        path=[user_A]
    if user_B in network[user_A][0] or user_B==user_A:
        path=path+[user_B]
        return path
    for i in network[user_A][0]:
        variable=path_to_friend(network,i,user_B,path)
        if i not in path and variable:
            path=path+[i]
            return variable
    return None