There is a polyline going through points (0, 0) – (*x*, *x*) – (2*x*, 0) – (3*x*, *x*) – (4*x*, 0) – ... - (2*kx*, 0) – (2*kx* + *x*, *x*) – .... 

We know that the polyline passes through the point (*a*, *b*). Find minimum positive value *x* such that it is true or determine that there is no such *x*.
Only one line containing two positive integers *a* and *b* (1 ≤ *a*, *b* ≤ 10<sup class="upper-index">9</sup>).
import sys
input = sys.stdin.readline

def sol(x) :
    return

a, b = map(int, input().split())
if b>a :
    print(-1)
elif a==b :
    print(a)
else :
    x = a+b
    while x>=1 :
        x /= 2
    print(x*2)