answer:-1
reason :
Once the numbers add up, once they decrease
what does it mean?
it mean that:
The first and second numbers we add are one, they become two
The second and third numbers are one and two. We subtract the second number from the third number, it becomes negative one and ...
1+1=2
1-2=-1
2+(-1)=1
-1-(+1)=-2
1+(-2)=-1
so we should - them
-2-(-1)=-1
