Sudoku GP Round 2- Killer Sudoku

Hi all,

I have been struggling with the Killer Sudoku for a long time now. I could place all the ones and have limited options in the four corners. How do I proceed? I tested both options for the corners, one of them led to the solution, the other to a contradiction, but it took quite a long time before that happened. I think there must be a more elegant logical path that I could not find. Any help would be very welcome. Thanks in advance!