Git Diff between two commits without including merge changes

How to take combined Git diff between 2 commit changesets without including the changes made by merges commits

For ex :

Commit-1a

File - gitdiff.js

Changes

+function check(){

+ var a=10;

+ console.log(a);

+}

Commit-2b (Merge commit)

File - gitdiff.js

Changes

+function merge(){

+ console.log(“inside merge”);

+}

+function mergechange(){

+ console.log(“inside mergechange”);

+}

Commit-3c

File - gitdiff.js

Changes

+function lastfn(){

+ console.log(“inside lastfn”);

+}

File Content after 3 commits

function merge(){

console.log(“inside merge”);

}

function check(){

var a=10;

console.log(a);

}

function mergechange(){

console.log(“inside mergechange”);

}

function lastfn(){

console.log(“inside lastfn”);

}

When I am giving git diff between commit 1a and commit 3c, I want the output diff like the following

Git diff commit-1a commit-3c

File - gitdiff.js

Changes

+function check(){

+ var a=10;

+ console.log(a);

+}

+function lastfn(){

+ console.log(“inside lastfn”);

+}

Is there anyway to get the output like the above without including the merge changes ?