import math
a, b = map(int,input().split())
min = math.floor(a / 0.08)
max = math.ceil((a+1)/0.08)
ans = 0
for i in range(min, max):
    if math.floor(i * 0.08) == a and math.floor(i*0.1) == b:
        ans = i
        break
if ans == 0:
    print("-1")
else:
    print(ans)