#include <iostream>
#include <cstring>
#include <algorithm>
#include<stack>
//车厢调度--栈
using namespace std;
const int MOD=1000000007;
const int N=10000+5;
const int dx[]= {-1,1,0,0};
const int dy[]= {0,0,-1,1};
using namespace std;
int a[N];
stack<int> S;
int main()
{
    int n;
    cin>>n;
    for(int i=1;i<=n;i++)//a[i]为到达B站的车厢
        cin>>a[i];

    int cur=1;//cur为需要进栈的车厢 
    for(int i=1;i<=n;i++)//进栈,到达A站；出栈,到达B站
    {
        while(cur<=a[i])//比a[i]小的车厢都要在栈中
            S.push(cur++);
        if(S.top()==a[i])//将a[i]弹出栈
            S.pop();
        else
        {
            cout<<"NO"<<endl;
            return 0;
        }
    }
    cout<<"YES"<<endl;
    return 0;
}