#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, t;
  cin >> n;
  set<long long int> s;
  while (n--) {
    cin >> t;
    s.insert(t);
  }
  if (s.size() >= 4)
    cout << "NO";
  else {
    if (s.size() == 3) {
      auto it1 = s.begin();
      auto it2 = it1;
      auto it3 = it2;
      it2++;
      it3++;
      it3++;
      if (2 * (*it2) == *it1 + *it3)
        cout << "YES";
      else
        cout << "NO";
    } else
      cout << "YES";
  }
}
