If value in column A is true or equal take the value from Column B [duplicate]

```Class       Grade
   Physics       8
   Math          7
   English       7.5
   Biology       5 
   Physics       4 

if(Class == Physics) {Sum(Grade)

I am looking that when the Class is similar to what I am searching it takes the Grade next to it. I assume something like this. and it will combine so in this case 12 for physics. And then That I can create a new dataframe where I can combine all the numbers corresponding to the Class.