#include<bits/stdc++.h>
using namespace std;

//#define debug(...)   printf( __VA_ARGS__ )
#define debug(...)   /****nothing****/

#define ll long long
#define ull unsigned long long
#define pb push_back
#define mem(arr,val)    memset(arr,val,sizeof(arr))
#define mp make_pair
#define pii pair<int,int>
#define F first
#define S second
#define sz(x) (int)(x).size()
#define nopos string::npos
#define fr(i,a,b) for(i=a;i<=b;i++)
#define frn(i,a,b) for(i=a;i>=b;i--)
int n,ar[2000],now,i,j,grt,k,fl=1,lft,pos;
stack<int> stk1,stk2;

int main()
{
    //freopen("d://i.txt","r",stdin);
    //freopen("d://o.txt","w",stdout);

    while(cin>>n && n!=0)
    {
        fr(i,0,n-1)
        {
            scanf("%d",ar+i) ;
            if(ar[i]==1)
                pos=i;
        }

        fr(i,0,pos-1)
         stk2.push(ar[i]);

        stk1.push(1);

        fr(i,pos+1,n-1)
         {
             while(!stk1.empty() && stk1.top()>ar[i])
             {
                 stk2.push(stk1.top());
                 stk1.pop();
             }

             stk1.push(ar[i]);
         }

//        print(stk1);
//        print(stk2);

        int lastneed=stk1.top()+1;

        while(!stk1.empty())
        {
           int val=stk1.top();

           stk1.pop();

           if(!stk1.empty())
             {
                 if(stk1.top()+1 !=val)
                 {
                     int need=stk1.top()+1;

                     while(!stk2.empty() && stk2.top()==need)
                     {
                         stk2.pop();
                         need++;
                         if(need==val)
                            break;
                     }

                     if(need!=val)
                     {
                         fl=0;
                         break;
                     }

                 }
             }

        }

        while(!stk2.empty() && stk2.top()==lastneed)
        {
            stk2.pop();
            lastneed++;
            if(lastneed==n+1)
                break;
        }

        if(lastneed!=(n+1) ) fl=0;

        printf((fl==0)?"no\n":"yes\n");

        fl=1;
        while(!stk2.empty()) stk2.pop();
        while(!stk1.empty()) stk1.pop();
    }

    return 0;

}