finish 2.1-4

2 files changed, 16 insertions, 1 deletions

diff --git a/README.md b/README.md
index 7827912..175e692 100644
--- a/README.md
+++ b/README.md
@@ -70,7 +70,7 @@ When you have completed this objective, you should be able to
 
 * [ ] Study Section 4.1 of the textbook.
 * [X] Study Section 2.1 of the textbook.
-* [ ] Do Exercise 2.1-3 from the textbook as part of Assignment 1.
+* [X] Do Exercise 2.1-3 from the textbook as part of Assignment 1.
 
 #### Learning Objective 1.2.2 – Analyzing Algorithms
 
diff --git a/notes.md b/notes.md
index b2e9cfc..4f5273b 100644
--- a/notes.md
+++ b/notes.md
@@ -345,3 +345,18 @@ return i
 * termination: terminate loop when the key matches the target value.
 
 2.1-4:
+
+Consider the problem of adding two `n-bit` binary integers, stored in two
+`n-element` arrays `A` and `B`. The sum of the two integers should be stored in
+binary form in an `(n + 1)-element` array `C`. State the problem formally and
+write pseudocode for adding two integers.
+
+Input: Two sequences of `n` numbers `A = {a1,a2,...aN}` and `B = {b1,b2,...,bN}`
+Output: A sequence `C` with `n+1` numbers such that `C = {a1+b1,a2+b2,...,aN+bN}`
+
+```plaintext
+i = 0
+max = 2 ** A.length
+for i < A.length
+  C[i] = A[i] + B[i]
+```