#include<bits/stdc++.h>
using namespace std;
const int N=1e5+10;
long long arr[N],b[N];
signed main(){
	long n,cnt=0;
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>arr[i];
		b[arr[i]]=i;
	}
	for(int i=1;i<=n;i++){
		if(arr[i]!=b[i]){
			cout<<"NO";
			return 0;
		}
	}
	cout<<"YES";
}