  We present a novel verification technique to prove interesting properties of
a class of array programs with a symbolic parameter N denoting the size of
arrays. The technique relies on constructing two slightly different versions of
the same program. It infers difference relations between the corresponding
variables at key control points of the joint control-flow graph of the two
program versions. The desired post-condition is then proved by inducting on the
program parameter $N$, wherein the difference invariants are crucially used in
the inductive step. This contrasts with classical techniques that rely on
finding potentially complex loop invaraints for each loop in the program. Our
synergistic combination of inductive reasoning and finding simple difference
invariants helps prove properties of programs that cannot be proved even by the
winner of Arrays sub-category from SV-COMP 2021. We have implemented a
prototype tool called diffy to demonstrate these ideas. We present results
comparing the performance of diffy with that of state-of-the-art tools.
