but only if you go halfway with me on that sandwich 的意思

最好给个完整句!
再问： "Hi, my name is JACK“ he said ,"and one kind person brought me this meal .I just finished eating and hate to waste good food .Can you help me out?"The old man answered gratefully ,"sure ,but only if you go halfway with me on that sandwith."
再答： only if。。。只有。。。才。。。 but only if you go halfway with me on that sandwich 不过那个三明治你要吃一半我才帮你