I have an issue with the following - 

I have a dataframe as below:

I want to move the values from df$C into df$A. 

I include df$B here as df$C only contains a value when df$B contains a string.

```
A      B       C

1234   NA      NA
NA     start   1500
2000   NA      NA
NA     end     2500
NA     NA      NA
NA     NA      NA
NA     start   3000

```

So, the desired result is as follows:

```
A     

1234  
1500     
2000   
2500   
NA     
NA     
3000     

```

I'd really appreciate your help with this!