I have a data frame that has two string characters stored in each row

```
s   ['64.0', '2']   
a   ['63.0', '2']   
b   ['63.0', '1']   

```

How to convert the first character string into numeric value,and omit the second character string,which results into data frame as follows :

```
s    64.0   
a    63.0
b    63.0   

```