#include<bits/stdc++.h>
using namespace std;
void print_vector(vector<int> array)
{
for(int i = 0, n = array.size(); i < n; i++)
cout<<array[i]<<" ";
cout<<endl;
}
int main()
{
int n = 0;
while(cin>>n)
{
if(n == 0)break;
vector<int> array(n);
stack<int> s1,s2;
for(int i = 0; i < n; i++)
cin>>array[i];
for(int i = 0; i < n - 1; i++)
{
if(array[i] > array[i+1])
{
s1.push(array[i]);
}
else if(!s1.empty())
{
if(array[i+1]>s1.top())
{
s2.push(s1.top());
s1.pop();
}
else
s2.push(array[i]);
}
else
s2.push(array[i]);
}
s2.push(array[n-1]);
while(!s1.empty())
{
s2.push(s1.top());
s1.pop();
}
bool flag = true;
int x = s2.top();
s2.pop();
while(!s2.empty())
{
if(x < s2.top())
{
flag = false;
}
x=s2.top();
s2.pop();
}
if(flag)
cout<<"yes\n";
else
cout<<"no\n";
}
}
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