I have a dataframe that looks like the following

```
df
     city   val
0   London   3
1   London   -1
2   London   -1
3   Paris    -5
4   Paris    -2
5   Rome     2
6   Rome     2

```

I want to select only the city that have at least one `val < 0`. I would like to have the following:

```
df
     city   val
0   London   3
1   London   -1
2   London   -1
3   Paris    -5
4   Paris    -2

```