package main

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

func main() {
	input := bufio.NewReader(os.Stdin)
	var num int

	_, err := fmt.Fscanf(input, "%d\n", &num)
	if err != nil {
	}

	var pillar = make([]int, num)
	var index, max int

	for i := 0; i < num; i++ {
		_, err := fmt.Fscanf(input, "%v", &pillar[i])
		if pillar[i] > max {
			max = pillar[i]
			index = i
		}
		if err != nil {
		}
	}

	for i, j := 1, 1; ; {
		left := 0
		right := 0
		if index-i >= 0 {
			left = pillar[index-i]
		}
		if index+j < num {
			right = pillar[index+j]
		}
		if index-i < 0 && index+j >= num {
			break
		}

		if left < max && left >= right {
			max = left
			i++
			continue
		}
		if right < max && right > left {
			max = right
			j++
			continue
		}
		fmt.Println("NO")
		return
	}
	fmt.Println("YES")

}