I am struggling with Python Pandas with groupby. How should I accomplish the following? For every fruit I would like to find the difference with the 'step 0' value of that fruit.

```
df = pd.DataFrame({'Fruit' : ['Apple', 'Apple', 'Apple', 'Banana', 'Banana', 'Banana'], 'Step' : [0, 1, 2, 0, 1, 2], 'Value' : [100, 102, 105, 200, 210, 195] })

    Fruit  Step  Value     to-be
0   Apple     0    100  -->  0
1   Apple     1    102  -->  2
2   Apple     2    105  -->  5
3  Banana     0    200  -->  0
4  Banana     1    210  --> 10
5  Banana     2    195  --> -5

```

Thank you!