#include<iostream>
#include<cmath>
using namespace std;
int n,a[100];
//zs
int main(){
	cin >> n;
	for (int i = 1; i <= n; i++) {
		cin >> a[i];
		if (a[i]-a[i-1] > 1 || a[i]-a[i-1] < -1){        
           cout<<"NO"<<endl;
            return 0;
        }
	}
		cout << "YES"<<endl;
	return 0;
}