n=int(input())
lsx=list(map(int,input().split()))
lsx.sort()
flg=0
for i in range(1,len(lsx)-1):
    s1=lsx[i]-lsx[i-1]
    s2=lsx[i+1]-lsx[i]
    if s1!=s2 or s1==0:
        flg=1
if flg==1:
    print("NO")
else:
    print("YES")