Let be a function defined on the set of all positive integers and having its values in the same set. Suppose that for all positive integers Find the possible value for
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Let $f(n)$ be a function defined on the set of all positive integers and having its values in the same set. Suppose that $f(f(n) + f(m)) = m + n$ for all positive integers $n,m.$ Find the possible value for $f(1988).$
Izvor: Međunarodna matematička olimpijada, shortlist 1988
Školjka 
be a function defined on the set of all positive integers and having its values in the same set. Suppose that 
for all positive integers 
Find the possible value for 