import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;

public class No406 {
	public static void main(String[] args) throws IOException{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(br.readLine());
		String[] str = br.readLine().split(" ");
		int[] nums = new int[n];
		for(int i=0; i < n; i++) nums[i] = Integer.parseInt(str[i]);
		Arrays.sort(nums);
		int difference = nums[1] - nums[0];
		int count = 0;
		if(difference != 0){
			for(int i=1; i< n-1; i++)
				if((nums[i+1] - nums[i]) == difference){
					count++;
				}
		}
		if(count == n-2) System.out.println("YES");
		else System.out.println("NO");
	}
}