Excel sheet choosing rows and columns and saving the values

Hi 

 

I want to:

 

1. Read data form particular excel sheets.

2. choose the particular rows and columns through programming and apply some formulas on it to get some values then put the values on the wanted cells

3. Then save all those values on the same excel sheets from which data was taken.

 

I am able to read the sheets.

 

But, with the remaining i need help. Also, i want to do it in C#

 

Thank you.