I have two dataframe of the same style and would like to merge them into a dataframe with the same columns whilst also combining one of the columns.

The two dataframes look like this:

```
    year variable
1  1968    2
2  1969    5
3  1970    <NA>
4  1971    <NA>
5  1972    <NA>

    year variable
1  1968    <NA>
2  1969    <NA>
3  1970    5
4  1971    7
5  1972    <NA>

```

and I would like to end up with a data frame that looks like this:

```
    year variable
1  1968    2
2  1969    5
3  1970    5
4  1971    7
5  1972    <NA>

```

Thanks in advance!