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After recapturing Sylla, the Company plans to establish a new secure system, a transferring net! The new system is designed as follows:
The Company staff choose N cities around the nation which are connected by "security tunnels" directly or indirectly. Once a week, Sylla is to be transferred to another city through the tunnels. As General ordered, the transferring net must reach a certain security level that there are at least 3 independent paths between any pair of cities a, b. When General says the paths are independent, he means that the paths share only a and b in common.
Given a design of a transferring net, your work is to inspect whether it reaches such security level.
The input consists of several test cases.
For each test case, the first line contains two integers, N ≤ 500 and M ≤ 20000. indicating the number of cities and tunnels.
The following M lines each contains two integers a and b (0 ≤ a, b < N), indicating the city a and city b are connected directly by a tunnel.
The input ends by two zeroes.
For each test case output "YES" if it reaches such security level, "NO" otherwise.
4 6 0 1 0 2 0 3 1 2 1 3 2 3 4 5 0 1 0 2 0 3 1 2 1 3 7 6 0 1 0 2 0 3 1 2 1 3 2 3 0 0
YES NO NO
| 时间上限 | 内存上限 |
| 5000 | 65536 |