#include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<stdio.h>
#include<cmath>
#include<vector>
#include<utility>
#define FOR(i, a, b) for(int i=(a);i<=(b);i++)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define MOD 1000000007

using namespace std;


int main(void) {
	int n;
	int *x;
	int point;
	bool flag = true;

	cin >> n;
	x = new int[n];
	FOR(i, 0, n - 1) {
		cin >> x[i];
	}
	point = abs(x[2] - x[0]);
	FOR(i, 1, n - 2) {
		if (abs(x[i - 1] - x[i + 1]) != point) {
			flag = false;
			break;
		}
	}
	sort(x, x + n );
	FOR(i, 0, n - 2) {
		if (x[i] == x[i + 1]) {
			flag = false;
			break;
		}
	}
	if (flag) {
		cout << "YES" << endl;
	}
	else {
		cout << "NO" << endl;
	}

	delete[] x;
	return 0;
}