package main

import . "fmt"
import . "sort"

func main() {
	var n int
	Scan(&n)
	a:=make([]int,n)
	for i:=range a {
		Scan(&a[i])
	}
	Ints(a)
	x:=a[0]!=a[n-1]
	for i,v:=range a[1:] {
		x=x&&(a[1]-a[0]==v-a[i])
	}
	if x {
		Println("YES")
	} else {
		Println("NO")
	}
}