CF584C Marina and Vasya

Marina loves strings of the same length and Vasya loves when there is a third string, different from them in exactly $ t $ characters. Help Vasya find at least one such string. More formally, you are given two strings $ s_{1} $ , $ s_{2} $ of length $ n $ and number $ t $ . Let's denote as $ f(a,b) $ the number of characters in which strings $ a $ and $ b $ are different. Then your task will be to find any string $ s_{3} $ of length $ n $ , such that $ f(s_{1},s_{3})=f(s_{2},s_{3})=t $ . If there is no such string, print $ -1 $ .

The first line contains two integers $ n $ and $ t $ ( $ 1

Print a string of length $ n $ , differing from string $ s_{1} $ and from $ s_{2} $ in exactly $ t $ characters. Your string should consist only from lowercase English letters. If such string doesn't exist, print -1.