# your code goes here
#406 birds same distance
N=int(input())
x=[int(i) for i in input().split()]
x.sort()
#print(x)
i=0
while  i<N-2 and x[i+1]-x[i]!=0 and x[i+2]-x[i+1]==x[i+1]-x[i]:
 #   print(x[i+2])
    i+=1
if i==N-2:
    print("YES")
else:
    print("NO")