Help me defeat my coworkers in the math puzzle of the month?!

The new school year has just started, and the head of the math department has started our monthly challenge again!. The puzzles are really put up for the kids, but all staff other than math teachers are allowed to participate!. There are about 5 staff members, myself included, who compete fiercely every month to be the first to turn in the correct answer!. I guess my skills got rusty over the summer break because this one has me stumped!. Can any math geniuses out there give me a hand with this one!?

If -
2 # 6 = 4
3 # 7 = 5
10 # 2 = 6
4 # 4 = 4

Then 13 # 12 = !?

What mathmatical operation does # stand for, and what is the answer to 13 # 12 =Www@Enter-QA@Com

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