#include <iostream>
using namespace std;
//G ANTON CANT SEE
int main() {
	int n;
	cin>>n;
	int a[n];
	for(int i=0;i<n;i++){
		cin>>a[i];
		//cout<<a[i]<<" ";
	}
	for(int i=0;i<n-1;i++){
	//	cout<<a[i]<<"   ";
		if(a[i]>a[i+1]) {
			cout<<"NO"<<endl;
			return 0;
		}
	}
	cout<<"YES"<<endl;
	return 0;
}