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

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

void main(void) {
	int n = 0;
	scanf("%d",&n);
	int numList[n];
	bool duckFlg = true;

	for(int i = 0; i < n ; i ++){
		scanf("%d",&numList[i]);
		for(int j = 0;j < i;j++){
			if(numList[i] == numList[j]){
				duckFlg = false;
			}
		}
	}
	if(duckFlg){
		size_t arraySize = sizeof(numList) / sizeof(numList[0]);
		qsort(numList, arraySize, sizeof(int), compareIntegers);
	}

	int equalInterval = numList[1] - numList[0];
	if(equalInterval != 0){
		for(int i = 1 ;i < n-1;i++){
			if( equalInterval != numList[i+1] - numList[i]){
				duckFlg = false;
				break;
			}
		}
	}else{
		duckFlg = false;
	}
	
	if(duckFlg){
		printf("YES");
	}else{
		printf("NO");
	}
}