Akbar teacher (Editorial)

Solution

The problem can be formulated to the following problem :

Let $a\le b$ then for which $x$ it holds :

thus a solution only exist if b-a is even which is x=\frac{b-a}{2} otherwise the answer is $-1$.

Complexity $\mathcal{O}(1)$.

for _ in range(int(input())):
    a,b=map(int,input().split())
    if(abs(a-b)%2==0):
        print((abs(a-b))//2)
    else:
        print(-1)