#include<bits/stdc++.h>
using namespace std;
int f[1010],head=0,z[1010];
int main()
{
freopen("train.in","r",stdin);
freopen("train.out","w",stdout);
int n;
cin>>n;
for(int i=1;i<=n;++i)
scanf("%d",&f[i]);
for(int h=1,k=1;h<=n;++h)
{
while(k<=f[h])
{
z[++head]=k;++k;
}
if(z[head]==f[h])
{
--head;
}
else
{
cout<<"NO";
return 0;
}
}
cout<<"YES";
fclose(stdin);fclose(stdout);
return 0;
}