#include <iostream>
#include <string.h>
using namespace std;
int v[27],n,i,x,z,k,p;
char a[100001],j;
int main()
{cin>>n;
k=0;
for(z=1;z<=26;z++)
v[i]=0;
if(n==1)
    cout<<"YES";
else
{
for(i=0;i<=n-1;i++){
    cin>>a[i];
    j=a[i];
x=(int)j;
v[x-96]++;
}
p=0;
for(i=1;i<=26;i++){
        if(v[i]>=2)
            p++;
}
if(p==0)
    k=1;
if(k==0)
    cout<<"YES";
    else
        cout<<"NO";
}
return 0;
}