#include<iostream>
#include<algorithm>
#include <vector>
#include<cmath>
using namespace std;
typedef long long int lont;
int main() {
	int N;
	cin >> N;
	vector<int>veca(N);
	vector<int>vecb(N - 1);
	vector<int>vecc(N - 2);
	for (int ia = 0; ia < N; ia++) {
		cin >> veca.at(ia);
	}
	sort(veca.begin(), veca.end());
	for (int ib = 0; ib < N - 1; ib++) {
		vecb.at(ib) = veca.at(ib + 1) - veca.at(ib);
	}
	int cnta = 0;
	for (int ie = 0; ie < N - 1; ie++) {
		if (vecb.at(ie) == 0) {
			cnta++;
		}
	}
	if (cnta)cout << "NO" << endl;
	else {
		for (int ic = 0; ic < N - 2; ic++) {
			vecc.at(ic) = vecb.at(ic + 1) - vecb.at(ic);
		}
		int cntb = 0;
		for (int id = 0; id < N - 2; id++) {
			if (vecc.at(id) != 0) {
				cntb++;
			}
		}
		if (cntb)cout << "NO" << endl;
		else cout << "YES" << endl;
	}
}