{"trustable":true,"prependHtml":"\u003cscript\u003e window.katexOptions \u003d { disable: true }; \u003c/script\u003e\n\u003cscript type\u003d\"text/x-mathjax-config\"\u003e\n MathJax.Hub.Config({\n tex2jax: {\n inlineMath: [[\u0027$$$\u0027,\u0027$$$\u0027], [\u0027$\u0027,\u0027$\u0027]],\n displayMath: [[\u0027$$$$$$\u0027,\u0027$$$$$$\u0027], [\u0027$$\u0027,\u0027$$\u0027]]\n }\n });\n\u003c/script\u003e\n\u003cscript async src\u003d\"https://mathjax.codeforces.org/MathJax.js?config\u003dTeX-AMS-MML_HTMLorMML\" type\u003d\"text/javascript\"\u003e\u003c/script\u003e","sections":[{"title":"","value":{"format":"HTML","content":"\u003cdiv class\u003d\"panel_content\"\u003eACM-DIY is a large QQ group where many excellent acmers get together. It is so harmonious that just like a big family. Every day,many \"holy cows\" like HH, hh, AC, ZT, lcc, BF, Qinz and so on chat on-line to exchange their ideas. When someone has questions, many warm-hearted cows like Lost will come to help. Then the one being helped will call Lost \"master\", and Lost will have a nice \"prentice\". By and by, there are many pairs of \"master and prentice\". But then problem occurs: there are too many masters and too many prentices, how can we know whether it is legal or not?\u003cbr\u003e\u003cbr\u003eWe all know a master can have many prentices and a prentice may have a lot of masters too, it\u0027s legal. Nevertheless,some cows are not so honest, they hold illegal relationship. Take HH and 3xian for instant, HH is 3xian\u0027s master and, at the same time, 3xian is HH\u0027s master,which is quite illegal! To avoid this,please help us to judge whether their relationship is legal or not. \u003cbr\u003e\u003cbr\u003ePlease note that the \"master and prentice\" relation is transitive. It means that if A is B\u0027s master ans B is C\u0027s master, then A is C\u0027s master.\u003c/div\u003e"}},{"title":"Input","value":{"format":"HTML","content":"The input consists of several test cases. For each case, the first line contains two integers, N (members to be tested) and M (relationships to be tested)(2 \u0026lt;\u003d N, M \u0026lt;\u003d 100). Then M lines follow, each contains a pair of (x, y) which means x is y\u0027s master and y is x\u0027s prentice. The input is terminated by N \u003d 0.\u003cbr\u003eTO MAKE IT SIMPLE, we give every one a number (0, 1, 2,..., N-1). We use their numbers instead of their names."}},{"title":"Output","value":{"format":"HTML","content":"For each test case, print in one line the judgement of the messy relationship.\u003cbr\u003eIf it is legal, output \"YES\", otherwise \"NO\"."}},{"title":"Sample","value":{"format":"HTML","content":"\u003ctable class\u003d\u0027vjudge_sample\u0027\u003e\n\u003cthead\u003e\n \u003ctr\u003e\n \u003cth\u003eInput\u003c/th\u003e\n \u003cth\u003eOutput\u003c/th\u003e\n \u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\u003cpre\u003e3 2\r\n0 1\r\n1 2\r\n2 2\r\n0 1\r\n1 0\r\n0 0\u003c/pre\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cpre\u003eYES\r\nNO\u003c/pre\u003e\u003c/td\u003e\n \u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n"}}]}