TypeError: unsupported operand type(s) for -: 'str' and 'str'

[Balaji]

Hi,
I am new to python. I am trying to do some operation with my data set

I am trying to do this operation

1	df['dif_and_sq']=(df['imdbRating']).sub((df['imdbVotes']))

i am getting this error

Error:
TypeError: unsupported operand type(s) for -: 'str' and 'str'

Even i tried to convert the variable to numeric but its not converting a it remains in serious

If I tried like this

1	df['dif_and_sq']=int(df['imdbRating']).sub(int(df['imdbVotes']))

The error is

Error:
TypeError: cannot convert the series to <class 'int'>

how to reslove this error