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CNT from IZhO 2010 2-nd round-1

Source:

September 4, 2010
combinatorics proposedcombinatorics

Problem Statement

Positive integers 1,2,...,n1,2,...,n are written on а blackboard (n>2n >2 ). Every minute two numbers are erased and the least prime divisor of their sum is written. In the end only the number 97 remains. Find the least nn for which it is possible.
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