# !/usr/bin/ python3.8
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
from collections import Counter

N = int(readline())
m = map(int, read().split())

degree = Counter(zip(m, m)).values()
print('YES' if max(degree) <= 2 else 'NO')