Is graph a tree

n = int(input())

adj = []
while n:
    adj.append(list(map(int, input().split())))
    n -= 1

visited = [0] * len(adj)
res = 'YES'
count = 0
for i in range(len(adj)):
    if not visited[i]:
        count += 1
        if count > 1:
            res = 'NO'
            break
        st = [(i, None)]
        visited[i] = 1
        while st:
            cur, parent = st.pop()
            for j in range(len(adj[cur])):
                if adj[cur][j]:
                    if not visited[j]:
                        st.append((j, cur))
                        visited[j] = 1
                    elif j != parent:
                        res = 'NO'
                        break
            if res == 'NO':
                break
    if res == 'NO':
        break

print(res)