#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main()
{
int n;
cin >> n;
vector<int> x(n);
for(int i = 0; i < n; i++){
cin >> x[i];
}
sort(x.begin(), x.end());
int start = x[1];
int inte = x[1]-x[0];
if(inte == 0){
cout << "NO" << endl;
} else {
int pos = start;
for(int i = 2; i < n; i++){
if(x[i] != pos+inte){
cout << "NO" << endl;
return 0;
}
}
cout << "YES" << endl;
}

return 0;
}