Xiao Ming bought two pens, red and blue, and shared 17 yuan. The unit prices of both pens are integers, and the red pen is more expensive than the blue pen. Xiaoqiang intends to use 35 yuan to buy these two kinds of pens (it is also allowed to buy only one of them), but no matter how he buys, he cannot use up 35 yuan exactly. So how much is the unit price of the red pen?
小明买红、蓝两支笔，共用了17元．两种笔的单价都是整数元，并且红笔比蓝笔贵．小强打算用35元来买这两种笔(也允许只买其中一种)，可是他无论怎么买，都不能把35元恰好用完．那么红笔的单价是多少元？
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