#include <bits/stdc++.h>
using namespace std;
int n;
int a[100005];
int slxh[100005]; /// slxh[t] = x có ý nghĩa là giá trị t được xuất hiện x lần
int dd = 1;
int main()
{
cin >> n;
for(int i = 1;i <= n;i++)
{
cin >> a[i];
}
for(int i = 1;i <= n;i++)
{
slxh[i] = 0;
}
for(int i = 1;i <= n;i++)
{
int t = a[i];
if(t < 1 || t > n)
{
cout <<"NO";
return 0;
}
slxh[t]++;
}
for(int i = 1;i <= n;i++)
{
if(slxh[i] != 1)
{
dd = 0;
break;
}
}
if(dd == 0)
{
cout <<"NO";
}
else cout <<"YES";
}
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