Negative cycle detection P37940

Write a program that, given a directed graph with positive and/or negative costs at the arcs, detects if there is a negative cycle in the graph.

Input

Input consists of several cases. Every case begins with the number of vertices $n$ and the number of arcs $m$. Follow $m$ triples $u, v, c$, indicating that there is an arc $u \to v$ of cost $c$, where $u \ne v$, $-10^6 \le c \le 10^6$. Assume $1 \le n \le 10^4$, $0 \le m \le 5n$, and that for every pair of vertices $u$ and $v$ there is at most one arc of the kind $u \to v$. All numbers are integers. Vertices are numbered from 0 to $n-1$.

Output

For every case, print "YES" if there is a negative cycle in the graph, and "NO" otherwise.