#include<iostream>
#include<vector>
#include<algorithm>
#include<functional>
#include<stack>
using namespace std;
int main() {
int n, tmp, i, j,count;
vector<int>a;
vector<int>sort_a;
vector<int>arr_empty;
stack<int>b;
while (true)
{
i = 0;
j = 0;
cin >> n;
if (n == 0) {
break;
}
a.clear();
sort_a.clear();
arr_empty.clear();
for (int i = 0; i < n; i++) {
cin >> tmp;
a.push_back(tmp);
sort_a.push_back(tmp);
}
sort(sort_a.begin(), sort_a.end());
while (j < n)
{
if (b.empty()) {
if (sort_a[i] != a[j]) {
b.push(a[j]);
j++;
}
else
{
arr_empty.push_back(a[j]);
i++;
j++;
}
}
else
{
if (sort_a[i] != a[j] && sort_a[i] != b.top()) {
b.push(a[j]);
j++;
}
else
{
if (sort_a[i] == a[j]) {
arr_empty.push_back(a[j]);
i++;
j++;
}
else
{
if (sort_a[i] == b.top()) {
arr_empty.push_back(b.top());
b.pop();
b.push(a[j]);
i++;
j++;
}
}
}
}
}
count = 0;
if (b.empty()) {
cout << "yes" << endl;
}
else
{
for (int tml = i; tml < n; tml++) {
if (sort_a[tml] != b.top()) {
count++;
b.pop();
}
else
{
arr_empty.push_back(b.top());
b.pop();
}
}
if (count > 0) {
cout << "no" << endl;
}
else
{
cout << "yes" << endl;
}
}
}
return 0;
}
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