import sys
input=lambda: sys.stdin.readline().rstrip()
n=int(input())
X=sorted([int(i) for i in input().split()])
d=X[1]-X[0]
if d==0:
  print("NO")
else:
  for i in range(n-2):
    if d==X[i+2]-X[i+1]:
      continue
    else:
      print("NO")
      break
  else:
    print("YES")