// No.406 鴨等間隔の法則
package main

import (
	"bufio"
	"fmt"
	"os"
	"sort"
	"strconv"
)

var sc = bufio.NewScanner(os.Stdin)
var wr = bufio.NewWriter(os.Stdout)

func next() string {
	sc.Scan()
	return sc.Text()
}

func nextInt() int {
	i, _ := strconv.Atoi(next())
	return i
}

func nextFloat() float64 {
	f, _ := strconv.ParseFloat(next(), 64)
	return f
}

func nextInts(n int) []int {
	ret := make([]int, n)
	for i := 0; i < n; i++ {
		ret[i] = nextInt()
	}
	return ret
}

func nextFloats(n int) []float64 {
	ret := make([]float64, n)
	for i := 0; i < n; i++ {
		ret[i] = nextFloat()
	}
	return ret
}

func nextStrings(n int) []string {
	ret := make([]string, n)
	for i := 0; i < n; i++ {
		ret[i] = next()
	}
	return ret
}

func main() {
	sc.Split(bufio.ScanWords)
	var n = nextInt()
	var x = nextInts(n)
	sort.Ints(x)
	d := max(x[1]-x[0], 1)
	ans := "YES"
	for i := 1; i < n; i++ {
		if (x[i-1] + d) != x[i] {
			ans = "NO"
			break
		}
	}
	fmt.Println(ans)
}