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nice combinatorics problem

Source: KGZ national 2016 2-problem

May 10, 2016

combinatorics

Problem Statement

The number

N

consists only

2's

and

1's

in its decimal representation.We know that,after deleting digits from N,we can get any number consisting

9999

1's

and

one

2's

in its decimal representation.Find the least number of digits in the decimal representation of N