Ok, let's see if this is any better:
Assume |S|=1
so -1\leqS\leq1
since 1\in\Re and \foralls\inS, 1\geqs, then by definition 1 is an upper bound
since 1\inS then 1=max(S)
Thus true for 1, now consider the case for m+1
Assume |S|=m+1
then -m-1\leqS\leqm+1
\forallm\inS, m+1>m and since...