listings has columns listing_id, city, neighbourhood and price.

Return a DataFrame with columns city and cheapest_price giving the lowest price in each city, ordered by city alphabetically, with the index renumbered from 0.

Input

   listing_id    city neighbourhood  price
0           1   Paris        Marais  180.0
1           2   Paris    Belleville   95.0
2           3  Lisbon          None  120.0
3           4  Lisbon        Alfama   70.0
4           5    Oslo   Grunerlokka  210.0
5           6    Oslo        Marais  150.0

Output

     city  cheapest_price
0  Lisbon            70.0
1    Oslo           150.0
2   Paris            95.0