finish 2.1-4

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@@ -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]
+```