import std.stdio, std.conv, std.string, std.array, std.range, std.algorithm, std.container;
import std.math, std.random, std.bigint, std.datetime, std.format;
void main(string[] args){ if(args.length > 1) if(args[1] == "-debug") DEBUG = 1; solve(); }
void log()(){ writeln(""); } void log(T, A ...)(T t, lazy A a){ if(DEBUG) write(t, " "), log(a); } bool DEBUG = 0; 
string read(){ static string[] ss; while(!ss.length) ss = readln.chomp.split; string res = ss[0]; ss.popFront; return res; }
// ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- //

void solve(){
	int n = read.to!int;
	int[] as;
	foreach(i; 0 .. n) as ~= read.to!int;
	
	int f = 0;
	int last = -10;
	foreach(a; as){
		if(last < a){
			if(f == 1){
				"NO".writeln;
				return;
			}
		}
		if(last > a){
			if(f == 0) f = 1;
		}
		last = a;
	}
	
	"YES".writeln;
	return;
	
}
