Given an array of 4 integers [a,b,c,d] representing two points (a, b) and (c, d), return the slope of the line joining these two points.
For an undefined slope (division by 0), return :undefined.
Assume that [a,b,c,d] and the answer are all integers (no floating numbers!). Slope: https://en.wikipedia.org/wiki/Slope
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