Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/shell/solutions/sum.md

@@ -0,0 +1,30 @@
+## Sum
+
+### Objectives
+
+1. Write a script that gets two numbers and prints their sum
+3. Make sure the input is valid (= you got two numbers from the user)
+2. Test the script by running and passing it two numbers as arguments 
+
+### Constraints
+
+1. Use functions
+
+### Solution
+
+```
+#!/usr/bin/env bash
+
+re='^[0-9]+$'
+
+if ! [[ $1 =~ $re && $2 =~ $re ]]; then
+    echo "Oh no...I need two numbers"
+    exit 2
+fi
+
+function sum {
+    echo $(( $1 + $2 ))
+}
+
+sum $1 $2
+```