#include"stdio.h"
#include"string.h"
int val[105000];
int father[105000];
int getfather(int k){
	if (k==father[k]) return k;
	int tmp=father[k];
	father[k]=getfather(father[k]);
	val[k]=val[k]^val[tmp];
	return father[k];
}
int main(){
	int n,m,i,e,t,xx,yy,a,b,x,y,k,tmp;
	int sign=0;
	scanf("%d %d",&n,&m);
	memset(val,0,sizeof(val));
    for (i=0;i<=n;i++) father[i]=i;
	for (e=1;e<=m;e++){
	  scanf("%d %d %d",&x,&y,&k);
	  x--;
	  if (x>y) {
	  	tmp=x;x=y;y=tmp;
	  }
	  xx=getfather(x);
	  yy=getfather(y);
	  a=val[x];
	  b=val[y];
	  if (xx!=yy){
	  	father[yy]=xx;
	  	val[yy]=k^a^b;
	  }else {
	  	if ((a^b)==k) ;
	  	else {sign=1;printf("%d\n",e);}
	  	
	  }
	}
	if (sign==0) printf("-1\n");
}