There are 1000 doors that are of the open-close type. When a person opens the door he closes it and then opens the other. When the first person goes he opens-closes the doors ion the multiples of 1 i.e., he opens and closes all the doors. When the second goes he opens and closes the doors 2, 4 6 8 rely. Similarly when the third one goes he does this for 3 6 9 12 15th doors rely. Find number of doors that are open at last.


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