Kata

Given an array of 4 integers
[a,b,c,d] representing two points (a, b) and (c, d), return a string representation of the slope of the line joining these two points.

For an undefined slope (division by 0), return undefined . Note that the "undefined" is case-sensitive.

   a:x1
   b:y1
   c:x2
   d:y2

Assume that [a,b,c,d] and the answer are all integers (no floating numbers!). Slope: https://en.wikipedia.org/wiki/Slope