Consider a post office that sells stamps in k different denominations c$1<$ c$2<$

c$3<...<$ ck

$,$ all positive integers. The problem is to find the smallest set of stamps

that total to a given cost n$.$ We have a greedy algorithm to do this, which uses the

highest value stamp that doesn$$t exceed the remaining amount, subtracts its value

from the remaining amount, and repeats until the remaining amount reaches zero.

Find a case where this algorithm fails.