#include <bits/stdc++.h>

using namespace std;

int main(void) {
  int n;
  cin >> n;

  vector<int> x(n);
  set<int> st;
  for (int i = 0; i < n; i++) {
    cin >> x[i];
    st.insert(x[i]);
  }
  sort(x.begin(), x.end());

  bool yes = (st.size() == n);
  int d = x[1] - x[0];
  for (int i = 2; i < n; i++) {
    yes &= x[i] - x[i - 1] == d;
  }

  cout << (yes ? "YES" : "NO") << endl;

  return 0;
}