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