```go
package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	reader := bufio.NewReader(os.Stdin)
	writer := bufio.NewWriter(os.Stdout)
	defer writer.Flush()

	var n int
	var k int64
	fmt.Fscan(reader, &n, &k)

	var s, t string
	fmt.Fscan(reader, &s)
	fmt.Fscan(reader, &t)

	var ans int64 = 0
	var diff int64 = 0

	for i := 0; i < n; i++ {
		diff = diff*2 + int64(t[i]-'a') - int64(s[i]-'a')
		if diff > k {
			diff = k
		}
		if diff+1 > k {
			ans += k
		} else {
			ans += diff + 1
		}
	}

	fmt.Fprintln(writer, ans)
}
```