joj 2141

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int a[10005],pos[10005];//pos::important 记录位置
int main()
{
    int  n;char ch;
    while(cin>>n&&n)
    {
        cin>>ch;
        memset(pos,0,sizeof(pos));
        for(int i=0;i<n;i++)
        {
            cin>>a[i];
            pos[a[i]]=i;
        }
        int flag=1;
        for(int i=0;i<n;i++)
        {
            for(int j=i+1;j<n;j++)
            {
                int t=a[j]+a[j]-a[i];
                if(t>=n||t<0) continue;
                if(pos[t]>j) {flag=0;break;}
            }
            if(flag==0) break;
        }
        if(flag) cout<<"yes"<<endl;
        else cout<<"no"<<endl;
    }
    return 0;
}