You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: 584C.go
    path of file: ./repos/codeforces-go/main/500-599
    
    the code of the file until where you have to start completion: package main

import (
	"bufio"
	. "fmt"
	"
    
func CF584C(in io.Reader, out io.Writer) {
	var n, t, d int
	s := [2]string{}
	Fscan(bufio.NewReader(in), &n, &t, &s[0], &s[1])
	ans := []byte(s[0])
	add := func(i int) { ans[i] = (ans[i]-'a'+1)%26 + 'a' }
	for i, v := range ans {
		if v != s[1][i] {
			d++
			add(i)
			if ans[i] == s[1][i] {
				add(i)
			}
		}
	}
	d = (d - t) * 2 // d<0 说明缺了，d>0 说明多了
	for i := range ans {
		if d < 0 && s[0][i] == s[1][i] {
			add(i) // 不一样
			d += 2
		} else if d > 0 && s[0][i] != s[1][i] {
			d--
			ans[i] = s[d&1][i] // 和其中一个一样
		}
	}
	if d == 0 {
		Fprint(out, string(ans))
	} else {
		Fprint(out, -1)
	}
}