Let a sequence with all its terms positive The positive integer which doesn't belong to the sequence is Find
%V0
Let $0<f(1)<f(2)<f(3)<\ldots$ a sequence with all its terms positive$.$ The $n-th$ positive integer which doesn't belong to the sequence is $f(f(n))+1.$ Find $f(240).$
Izvor: Međunarodna matematička olimpijada, shortlist 1978
Školjka 
a sequence with all its terms positive
The 
positive integer which doesn't belong to the sequence is 
Find 