int@n,@m,@s,@t,@k,@(a,b)[m];
if(m==0)exit(1);
graph g;
g.setEdge(n+1,m,a,b);
ll l=g.getDist(s,t);
wt(n==1?"No":l<0?n==2&&~k&1?"No":"Unknown":l+k&1?"No":l>k||!l&&!g.es[s]?"Unknown":"Yes");