#include<iostream>
#include<cstdio>
#include<cmath>
#include<algorithm>
#include<cstring>
int n,tot,top,a[1005],stack[1005];
using namespace std;
int main(void)
{
    //freopen("test.in","r",stdin);
    //freopen("test.input","w",stdinput);
    cin>>n;
    for(int i=1;i<=n;++i)
    {
        cin>>a[i];
    }
    for(int i=1;i<=n;++i)
    {
        if(a[i]<=tot)
        {
            if(a[i]==stack[top])
            {
                --top;
            }
            else
            {
                cout<<"NO"<<endl;
                return 0;
            }
        }
        else
        {
            for(int j=tot+1;j<=a[i];++j)
            {
                stack[++top]=j;
            }
            --top;
            tot=a[i];
        }
    }
    cout<<"YES"<<endl;
    return 0;
}