-20+-15+-12+-1+12+15+20
Don't do it left to right, cancel them in pairs.
Just lots of adding and subtracting. Take it step by step by starting from left to right.
-20 - 15 = -35
-35 - 12 = ?
OR
you could start cancelling opposite terms like + 15 and - 15 and end up with the answer faster.
Answer is basically $$-{\mathtt{1}}$$
+4473 
