#!/usr/bin/env python3


import numpy


def main():
    _ = int(input())
    xs = numpy.fromiter((int(x) for x in input().split()), dtype=numpy.int32)
    xs.sort()
    ds = numpy.diff(xs)
    print("YES" if ds[0] > 0 and numpy.all(ds == ds[0])  else "NO")


if __name__ == "__main__":
    main()