---

package main

import (
	"fmt"
)

func main() {
	var s string
	fmt.Scanf("%v", &s)

	begA := 0
	for i := 0; i < len(s); i++ {
		if s[i] == 'a'{
			begA++
		} else {
			break
		}
	}

	endA := 0
	for i := len(s) - 1; i >= 0; i-- {
		if s[i] == 'a'{
			endA++
		} else {
			break
		}
	}

	ph := true
	i := begA
	j := len(s) - endA - 1
	for ; i < j; {
		if s[i] != s[j] {
			ph = false
			break
		}
		i++
		j--
	}

	if !ph || begA > endA {
		fmt.Println("No")
	} else {
		fmt.Println("Yes")
	}
}