#include <bits/stdc++.h>

using namespace std;

int main(void)
{
	cin.tie(0);
	ios::sync_with_stdio(false);

	int n, t;
	vector <int> v;

	cin >> n;

	for (int i = 0; i < n; i++)
	{
		cin >> t;
		v.push_back(t);
	}

	sort(v.begin(), v.end());

	for (int i = 1; i < n; i++)
	{
		if (v[i - 1] == v[i])
		{
			cout << "NO" << '\n';
			return 0;
		}
	}

	for (int i = 1; i < n - 1; i++)
	{
		if (abs(v[i] - v[i - 1]) == abs(v[i + 1] - v[i]))
		{
			cout << "NO" << '\n';
			return 0;
		}
	}

	cout << "YES" << '\n';
	return 0;
}