There are some 6-inch and 7-inch long wooden sticks. Zhang Hua took some of them and joined them together to get many wooden sticks of different lengths (not counting the joints). What is the number of inches that cannot be obtained from the sticks of the following lengths
有一些6寸和7寸长的木棍，张华同学从中取出一些接在一起，可以得到许多长度不同的木棍(不计接头处)．则下列长度的木棍中不能得到的是多少寸
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