package main

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

func main() {
    var n int
    fmt.Scan(&n)
    x := make([]int, n)
    sc := bufio.NewScanner(os.Stdin)
    sc.Split(bufio.ScanWords)
    for i := 0; i < n; i++ {
        sc.Scan()
        x[i], _ = strconv.Atoi(sc.Text())
    }
    sort.Ints(x)
    flag := true
    diff := x[1] - x[0]
    if (diff == 0) {
        flag = false
    }
    for i := 2; i < n && flag; i++ {
        if (x[i] - x[i-1] != diff) {
            flag = false
        }
    }
    if flag {
        fmt.Println("YES")
    } else {
        fmt.Println("NO")
    }
}