Quad Sum Sudoku 2013 - 1

Initially I had thought of posting a Killer Sudoku today for the GP Practice. Then I realised that a Killer Sudoku would be readily available to all. So I decided to create a Quad Sum Sudoku.

This is of an Easy-Medium Difficulty level and should not take more than 5 minutes for an average solver. Top solvers should easily be able to crack it in under 3 minutes.

Rules of Classic Sudoku apply. Additionally, a circle at a corner implies that 1 digit is the sum of the other 3 digits around that corner.