For natural number n, define the function f(n) to be the number you get by adding the digits of the number n. For example, f(16) = 7, f(f(78)) = 6, and f(f(f(5978))) = 2. Find the least natural number n such that f(f(f(n))) is not a one-digit number.

Source: Purple Comet Math Meet, High School Round, 2008

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