----9

--5---4

-1-----3

8--2--6--7

start with the vertex.. then try to get the sum.. you should notice that s1, s2=s1 + 1, and s3 = s1 + 2.. then for the remaining numbers, try to create pairs which is sumthing like that, i min, n1, then n2 = n1 + 1, n3 = n1 + 2.. by here, you should know which side to put them..