N = input()
List = []
w = raw_input()
w_split = w.split()
for i in range(0,N):
    List.append(int(w_split[i]))
List.sort()

A = List
B = A[:]
LL =[]
for i in range(0,N-1):
    LL.append(abs(B[i]-A[i+1]))
if len(set(LL)) == 1 and len(List) == len(set(List)):
    print 'YES'
else:
    print 'NO'