I have a df structured like this, but with a large number of columns and rows: 

```
 A   B   C   D 
 1   4   3   3

```

and I want to obtain a df with a single column, made by the concatenation of all the previous elements:

```
 A
 1
 B
 4
 C
 3
 D
 3

```

How can I do?
Better if solutions comprise the use of dplyr.