#include<bits/stdc++.h>
using namespace std;
int a[101010];
int main()
{
int i,j,k,m,n;
cin>>n;
for(i=1; i<=n; i++)
{
cin>>a[i];
}
int maxx=-1,minn=1e9+10,flag=0;
for(i=1; i<=n; i++)
{
if(a[i]>=maxx&&(a[i]<=a[i+1]||a[i]>=minn))
maxx=a[i];
// a[i]=0;
else if(minn>=a[i])
{
minn=a[i];
// a[i]=1;
}
else
{
flag=1;
break;
}
}
if(flag)
printf("no\n");
else
printf("yes\n");
}