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:
def find_slope(points):
    run = points[0] - points[2]
    rise = points[1] - points[3]
    return "undefined" if run == 0 else "%d" % (rise / run)
def find_slope(points):
    run = points[0] - points[2]
    rise = points[1] - points[3]
    return "undefined" if run == 0 else "%d" % (rise / run)