What can be added to - 1 so it can be a positive 1?

So represent missing number with x

x means unknown so

x+-1=+1

adding a negative means subtraction

x-1=1

you can do anything to an equation as long as you do it to both sides

add 1 to both sides

x+1-1=1+1

x+0=2

x=2

the number is 2