// yukicoder: No.406 鴨等間隔の法則
// 2019.4.16 bal4u

#include <stdio.h>
#include <stdlib.h>

//// 高速入力
#if 1
#define gc() getchar_unlocked()
#else
#define gc() getchar()
#endif

int in()    // 非負整数の入力
{
	int n = 0, c = gc();
	do n = 10 * n + (c & 0xf), c = gc(); while (c >= '0');
	return n;
}

int N;
int x[100005];

int cmp(const void *a, const void *b) {	return *(int *)a - *(int *)b; }

int main()
{
	int i, d;

	N = in();
	for (i = 0; i < N; i++) x[i] = in();
	qsort(x, N, sizeof(int), cmp);
	d = x[1] - x[0]; 
	if (d == 0) puts("NO");
	else {
		for (i = 2; i < N; i++) if (x[i] - x[i - 1] != d) break;
		puts(i >= N ? "YES" : "NO");
	}
	return 0;
}