#include<bits/stdc++.h> #define ll long long using namespace std; ll a,b,c=0,i=1,j; int main(){ cin>>a; stackq; q.push(0); for(;i<=a;i++){ cin>>b; if(q.top()>b){ cout<<"NO"; return 0; } else{ for(;j<=b;j++)q.push(j); q.pop(); } } cout<<"YES"; }