As an acmer, Devil Aguin particularly loves numbers. This time, with a sequence consisting of $n$ elements $1 \sim n$ initially, Devil Aguin asks you to process the sequence until all the elements in it turn to zero.
What you can do in one operation are as following :
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Select some elements from the sequence arbitrarily.
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Select a positive integer $x$ arbitrarily.
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Subtract $x$ from the elements you select.
It is obvious that there are various methods to make elements of the sequence turn to zero. But Devil Aguin demands of you to use the minimum operations. Please tell him how many operations you will use.