#include<bits/stdc++.h>
using namespace std;

int main(){
    int N,in;
    vector<int> v;

    cin >> N;
    while(cin >> in) v.push_back(in);
    sort(v.begin(),v.end());

    for(int i=0;i<N-2;i++){
        if(v[i+1]-v[i]!=v[i+2]-v[i+1] || v[i+1]-v[i]==0 || v[i+2]-v[i+1]==0){
            cout << "NO" << endl;
            return 0;
        }
    }
    cout << "YES" << endl;

    return 0;
}