#include <iostream>
#include <string.h>
#include<vector>
#include<queue>
#include<stack>
#include<array>

using namespace std;

int sum1(int a) {
	int l = 0;
	while (a != 0) {
		l = l + a % 10;
		a = a / 10;
		if (l >= 10)
			l = l / 10;
	}
	return l;
}

string get() {
	int n;
	long long int*h=new long long int[100005];
	cin >> n;
	int t;
	for (int i = 0;i < n;i++)
	{
		cin >> h[i];
		
	}

	for (int i = n - 1;i > 0;i--) {
		if (h[i] < h[i - 1])
		return "NO";
	}
	return "YES";
}


int main() {
	
	cout<<get();
	return 0;
}