The next letter you can solve for is the letter O in the middle and bottom rows. The letter S has to be either 8 or 9, since you are adding the number 1 to it, and it has to give you at least an answer of 10 so that you can carry over a 1 to the next left column. If S were an 8, and you are adding a 1 to it, the most letter O could be is the number 0. If S were a 9, and you are adding a 1 to it, then the letter O could be the numbers 0 or 1. However, 1 is already taken by the letter M. That leaves the number 0 as the only possible solution for the letter O.
At this point, the solution would look like this:
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